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/dbauthz/dbauthz.go

@@ -2636,6 +2636,14 @@ func (q *querier) GetChatMessageByID(ctx context.Context, id int64) (database.Ch
 	return msg, nil
 }
 
+func (q *querier) GetChatMessageSummariesPerChat(ctx context.Context, createdAfter time.Time) ([]database.GetChatMessageSummariesPerChatRow, error) {
+	// Telemetry queries are called from system contexts only.
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceSystem); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatMessageSummariesPerChat(ctx, createdAfter)
+}
+
 func (q *querier) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	// Authorize read on the parent chat.
 	_, err := q.GetChatByID(ctx, arg.ChatID)
@@ -2684,6 +2692,14 @@ func (q *querier) GetChatModelConfigs(ctx context.Context) ([]database.ChatModel
 	return q.db.GetChatModelConfigs(ctx)
 }
 
+func (q *querier) GetChatModelConfigsForTelemetry(ctx context.Context) ([]database.GetChatModelConfigsForTelemetryRow, error) {
+	// Telemetry queries are called from system contexts only.
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceSystem); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatModelConfigsForTelemetry(ctx)
+}
+
 func (q *querier) GetChatProviderByID(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
 	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
 		return database.ChatProvider{}, err
@@ -2800,6 +2816,14 @@ func (q *querier) GetChatsByWorkspaceIDs(ctx context.Context, ids []uuid.UUID) (
 	return fetchWithPostFilter(q.auth, policy.ActionRead, q.db.GetChatsByWorkspaceIDs)(ctx, ids)
 }
 
+func (q *querier) GetChatsUpdatedAfter(ctx context.Context, updatedAfter time.Time) ([]database.GetChatsUpdatedAfterRow, error) {
+	// Telemetry queries are called from system contexts only.
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceSystem); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatsUpdatedAfter(ctx, updatedAfter)
+}
+
 func (q *querier) GetConnectionLogsOffset(ctx context.Context, arg database.GetConnectionLogsOffsetParams) ([]database.GetConnectionLogsOffsetRow, error) {
 	// Just like with the audit logs query, shortcut if the user is an owner.
 	err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceConnectionLog)

coderd/database/dbauthz/dbauthz_test.go

@@ -4012,6 +4012,20 @@ func (s *MethodTestSuite) TestSystemFunctions() {
 		dbm.EXPECT().GetWorkspaceAgentsCreatedAfter(gomock.Any(), ts).Return([]database.WorkspaceAgent{}, nil).AnyTimes()
 		check.Args(ts).Asserts(rbac.ResourceSystem, policy.ActionRead)
 	}))
+	s.Run("GetChatsUpdatedAfter", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		ts := dbtime.Now()
+		dbm.EXPECT().GetChatsUpdatedAfter(gomock.Any(), ts).Return([]database.GetChatsUpdatedAfterRow{}, nil).AnyTimes()
+		check.Args(ts).Asserts(rbac.ResourceSystem, policy.ActionRead)
+	}))
+	s.Run("GetChatMessageSummariesPerChat", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		ts := dbtime.Now()
+		dbm.EXPECT().GetChatMessageSummariesPerChat(gomock.Any(), ts).Return([]database.GetChatMessageSummariesPerChatRow{}, nil).AnyTimes()
+		check.Args(ts).Asserts(rbac.ResourceSystem, policy.ActionRead)
+	}))
+	s.Run("GetChatModelConfigsForTelemetry", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatModelConfigsForTelemetry(gomock.Any()).Return([]database.GetChatModelConfigsForTelemetryRow{}, nil).AnyTimes()
+		check.Args().Asserts(rbac.ResourceSystem, policy.ActionRead)
+	}))
 	s.Run("GetWorkspaceAppsCreatedAfter", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		ts := dbtime.Now()
 		dbm.EXPECT().GetWorkspaceAppsCreatedAfter(gomock.Any(), ts).Return([]database.WorkspaceApp{}, nil).AnyTimes()