fix(coderd): give chat message ids an append-order guarantee (#27495)

Chat message ordering was derived from `created_at`, which is `now()`
and therefore the transaction start time. That makes it unusable as an
append-order column for two independent reasons: every row in one
`InsertChatMessages` batch shares a single timestamp, and two concurrent
transactions can commit in the opposite order to the one they started
in.

This PR gives `chat_messages.id` a real append-order guarantee and moves
the history reads onto it.

## Changes

**`InsertChatMessages` had no input-order guarantee.** Callers index the
returned slice by input position. That only worked because PostgreSQL
happens to evaluate the `BIGSERIAL` default in row order. Ids are now
allocated up front and the k-th smallest is assigned to input index k,
so the pairing does not depend on where the column default is evaluated.
Returned rows are explicitly `ORDER BY id`.

**Three history reads now order by `id`.**

| Query | Was | Now |
|---|---|---|
| `GetChatMessagesByChatID` | `created_at ASC` | `id ASC` |
| `GetChatMessagesByRevisionForStream` | `created_at ASC, id ASC` | `id
ASC` |
| `GetLastChatMessageByRole` | `created_at DESC, id DESC` | `id DESC` |

`GetChatMessagesByChatID` paginated by `id` while ordering by
`created_at`, which is incoherent on its own terms.

The other two matter because of who consumes them. The stream query
supplies incremental updates on the same socket that emits a full
`GetChatMessagesByChatID` snapshot on history reset, so once that
snapshot moved to `id` the two disagreed under timestamp skew.
`GetLastChatMessageByRole` returns an id that is then used as an id
cursor, both as `AfterID` when synthesizing tool cancellations and as
`chats.last_read_message_id`, where a stale anchor leaves later
assistant messages permanently unread.

A tie-breaker would not have fixed either one. It only resolves equal
timestamps; leading with `created_at` is the actual defect.

**`GetLastChatMessageByRole` loses its index, so this adds one.** `ORDER
BY created_at DESC, id DESC` could take an ordered scan of
`idx_chat_messages_chat_created`. Nothing in the schema can supply
`ORDER BY id DESC LIMIT 1` for a given `chat_id` and `role`, so the
planner switches to a backward scan of the primary key and filters every
newer row in the table, scanning all of it when the chat has no message
in that role, which is the routine case for a fresh chat. Migration
`000559` adds `(chat_id, role, id DESC) WHERE deleted = false`, the same
shape as the existing `idx_chat_messages_user_prompts`. This matters
because the query is hot: it runs on every stream connect and
disconnect, and once per turn when synthesizing tool cancellations.

`GetChatMessagesForPromptByChatID` has the same defect and is fixed in
the stacked PR, because its compaction boundary change is semantic and
deserves a separate review. Auto-archive stays timestamp-based
deliberately: it measures activity, not order.

Wrapping the insert in a CTE (needed because `INSERT` cannot take `ORDER
BY`) makes sqlc synthesize `InsertChatMessagesRow`. It is structurally
identical to `ChatMessage`, so the call sites use a direct struct
conversion that stops compiling if the two ever diverge.

## Testing

Behavior tests write `created_at` values inverted against id order, so a
reader that leads with `created_at` returns the batch backwards. All
three queries were verified red by reverting the `ORDER BY` and
regenerating: the stream query returned `[3,2,1]` for `[1,2,3]`, and
`GetLastChatMessageByRole` picked id 1 instead of id 3.

`TestInsertChatMessagesOrderContract` asserts against the generated SQL,
covering what a behavior test cannot: PostgreSQL evaluates the id
default in row order anyway, so a batch still looks ordered once the
guarantee is removed.

`TestChatMessagesSequenceCacheIsOne` guards the cross-batch half of the
invariant. Ids follow chat row lock order only while the sequence hands
out one value at a time; sequence cache blocks are per session, so with
a cache above one a backend holding stale cached values can lock second
and still commit lower ids. Bumping a sequence cache is an ordinary
throughput tweak, and it would silently corrupt history order.

The index was checked on a 200k row fixture. Without it, the zero-match
lookup filters all 200,000 rows over 2763 buffers; with it, the plan is
an index scan with both `chat_id` and `role` in the index condition, no
sort node, and 3 buffers.

Note that the within-batch mapping does not depend on the cache size. It
is established by `ROW_NUMBER() OVER (ORDER BY id)` over the allocated
ids, so it holds regardless of `nextval` evaluation order.

## Note on the deleted subagent hand-sort

The subagent history reader's hand-sort stays deleted, but calling it
redundant was imprecise. It sorted by `created_at` then `id`, so it is
only equivalent to `id` ordering when the two agree. When they disagree
the old code selected a different "latest assistant". This is a
deliberate behavior change to match the new invariant, not dead-code
removal.

> Opened by Mux on behalf of Mike.

coderd/database/querier.go

@@ -461,9 +461,13 @@ type sqlcQuerier interface {
 	// after the given timestamp. Uses message created_at so that
 	// ongoing activity in long-running chats is captured each window.
 	GetChatMessageSummariesPerChat(ctx context.Context, createdAfter time.Time) ([]GetChatMessageSummariesPerChatRow, error)
+	// Ordered by id to match the @after_id cursor. created_at is the transaction
+	// start time, so it can disagree with append order when a transaction takes the
+	// chat row lock later than one that started after it.
 	GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error)
 	GetChatMessagesByChatIDAscPaginated(ctx context.Context, arg GetChatMessagesByChatIDAscPaginatedParams) ([]ChatMessage, error)
 	GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg GetChatMessagesByChatIDDescPaginatedParams) ([]ChatMessage, error)
+	// Stream deltas and reset snapshots must use the same message order.
 	GetChatMessagesByRevisionForStream(ctx context.Context, arg GetChatMessagesByRevisionForStreamParams) ([]ChatMessage, error)
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
@@ -637,6 +641,8 @@ type sqlcQuerier interface {
 	// param created_at_opt: The created_at timestamp to filter by. This parameter is usd for pagination - it fetches notifications created before the specified timestamp if it is not the zero value
 	// param limit_opt: The limit of notifications to fetch. If the limit is not specified, it defaults to 25
 	GetInboxNotificationsByUserID(ctx context.Context, arg GetInboxNotificationsByUserIDParams) ([]InboxNotification, error)
+	// The returned id becomes both an AfterID cursor and last_read_message_id, so
+	// "last" must use id order.
 	GetLastChatMessageByRole(ctx context.Context, arg GetLastChatMessageByRoleParams) (ChatMessage, error)
 	GetLastUpdateCheck(ctx context.Context) (string, error)
 	GetLatestCryptoKeyByFeature(ctx context.Context, feature CryptoKeyFeature) (CryptoKey, error)
@@ -1105,7 +1111,10 @@ type sqlcQuerier interface {
 	// with concurrent FinalizeStale under READ COMMITTED isolation.
 	InsertChatDebugStep(ctx context.Context, arg InsertChatDebugStepParams) (ChatDebugStep, error)
 	InsertChatFile(ctx context.Context, arg InsertChatFileParams) (InsertChatFileRow, error)
-	InsertChatMessages(ctx context.Context, arg InsertChatMessagesParams) ([]ChatMessage, error)
+	// Returns the inserted rows in input array order. Ids are allocated before the
+	// insert and the k-th smallest is assigned to input index k, so callers may
+	// index the result positionally.
+	InsertChatMessages(ctx context.Context, arg InsertChatMessagesParams) ([]InsertChatMessagesRow, error)
 	InsertChatModelConfig(ctx context.Context, arg InsertChatModelConfigParams) (ChatModelConfig, error)
 	// Legacy queue insertion path. When no caller-supplied creator exists,
 	// preserve the created_by invariant by attributing the queued row to the