feat(coderd): add telemetry for agents chats and messages (#24068)

Adds telemetry collection for the agents chat system (`/agents`) to the
existing telemetry snapshot pipeline.

Three new snapshot fields:
- **`Chats`** — per-chat metadata (id, owner, status, mode,
workspace_id, root_chat_id, has_parent, archived, model config)
collected time-windowed via `createdAfter`
- **`ChatMessageSummaries`** — per-chat aggregated message metrics
(counts by role, token sums by type, cost, runtime, model count,
compression count) collected time-windowed
- **`ChatModelConfigs`** — model configuration metadata (provider,
model, context limit, enabled, default) collected as full dump

No PII is included — titles, message content, and URLs are excluded at
the SQL level. Only structural metadata flows through telemetry.

<details><summary>Implementation plan</summary>

### SQL Queries (`coderd/database/queries/chats.sql`)
- `GetChatsCreatedAfter` — time-windowed chat metadata
- `GetChatMessageSummariesPerChat` — per-chat message aggregates via
`GROUP BY`
- `GetChatModelConfigsForTelemetry` — full dump of model configs

### Telemetry (`coderd/telemetry/telemetry.go`)
- `Chat`, `ChatMessageSummary`, `ChatModelConfig` structs
- `ConvertChat`, `ConvertChatMessageSummary`, `ConvertChatModelConfig`
conversion functions
- Three `eg.Go()` blocks in `createSnapshot()` following the existing
collection pattern

### Authorization (`coderd/database/dbauthz/dbauthz.go`)
- System-only access for all three queries via `rbac.ResourceSystem`

### Tests
- `TestChatsTelemetry` in `coderd/telemetry/telemetry_test.go` — creates
chats (root + child), messages with token/cost data, model configs;
verifies all snapshot fields
- dbauthz test entries for all three queries in
`coderd/database/dbauthz/dbauthz_test.go`

</details>

> 🤖 Generated by Coder Agents

coderd/database/querier.go

@@ -271,12 +271,18 @@ type sqlcQuerier interface {
 	// otherwise the setting defaults to true.
 	GetChatIncludeDefaultSystemPrompt(ctx context.Context) (bool, error)
 	GetChatMessageByID(ctx context.Context, id int64) (ChatMessage, error)
+	// Aggregates message-level metrics per chat for messages created
+	// 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)
 	GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error)
 	GetChatMessagesByChatIDAscPaginated(ctx context.Context, arg GetChatMessagesByChatIDAscPaginatedParams) ([]ChatMessage, error)
 	GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg GetChatMessagesByChatIDDescPaginatedParams) ([]ChatMessage, error)
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
 	GetChatModelConfigs(ctx context.Context) ([]ChatModelConfig, error)
+	// Returns all model configurations for telemetry snapshot collection.
+	GetChatModelConfigsForTelemetry(ctx context.Context) ([]GetChatModelConfigsForTelemetryRow, error)
 	GetChatProviderByID(ctx context.Context, id uuid.UUID) (ChatProvider, error)
 	GetChatProviderByProvider(ctx context.Context, provider string) (ChatProvider, error)
 	GetChatProviders(ctx context.Context) ([]ChatProvider, error)
@@ -304,6 +310,10 @@ type sqlcQuerier interface {
 	GetChatWorkspaceTTL(ctx context.Context) (string, error)
 	GetChats(ctx context.Context, arg GetChatsParams) ([]GetChatsRow, error)
 	GetChatsByWorkspaceIDs(ctx context.Context, ids []uuid.UUID) ([]Chat, error)
+	// Retrieves chats updated after the given timestamp for telemetry
+	// snapshot collection. Uses updated_at so that long-running chats
+	// still appear in each snapshot window while they are active.
+	GetChatsUpdatedAfter(ctx context.Context, updatedAfter time.Time) ([]GetChatsUpdatedAfterRow, error)
 	GetConnectionLogsOffset(ctx context.Context, arg GetConnectionLogsOffsetParams) ([]GetConnectionLogsOffsetRow, error)
 	GetCryptoKeyByFeatureAndSequence(ctx context.Context, arg GetCryptoKeyByFeatureAndSequenceParams) (CryptoKey, error)
 	GetCryptoKeys(ctx context.Context) ([]CryptoKey, error)