feat: add chat suffix messages, idle failure, and content update support (#27428)

Adds generic chat state and query capabilities that the lifecycle hooks
integration (#27429) builds on. Part of the lifecycle hooks stack
(#27401, #27429, #27430).

- `chatstate`: `EditMessage` accepts caller-provided suffix messages
inserted after the replacement in the same transaction, transitions can
carry a typed error kind, and `FinishError` is also allowed from waiting
chats so admission-time failures can park an idle chat in error.
- `chatstate`: `ValidateToolResults` holds the submitted-tool-result
rules (duplicate, invalid JSON, missing, unexpected) in one place, so
`CompleteRequiresAction` and API-level prechecks reject the same
payloads with the same typed causes.
- `database`: `InsertChat` accepts an optional caller-provided ID.

No hook-specific state or behavior is introduced here; these primitives
are usable by any caller.

An earlier revision added a message-content rewrite primitive so a
`pre_tool_use` override could update an already-committed tool call.
Message content is immutable by design, and @hugodutka pushed back on
changing that. The rewrite is gone: #27429 now dispatches the hook
before the assistant message is stored, so the stored input is the one
that runs and nothing needs updating.

> This PR was written by Mux, an AI coding agent, on Mike's behalf.

coderd/database/dbgen/dbgen.go

@@ -93,6 +93,7 @@ func Chat(t testing.TB, db database.Store, seed database.Chat) database.Chat {
 	}
 
 	chat, err := db.InsertChat(genCtx, database.InsertChatParams{
+		ID:                uuid.NullUUID{UUID: seed.ID, Valid: seed.ID != uuid.Nil},
 		OrganizationID:    takeFirst(seed.OrganizationID, uuid.New()),
 		OwnerID:           takeFirst(seed.OwnerID, uuid.New()),
 		WorkspaceID:       seed.WorkspaceID,