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/dbmetrics/querymetrics.go

@@ -1176,6 +1176,14 @@ func (m queryMetricsStore) GetChatMessageByID(ctx context.Context, id int64) (da
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatMessageSummariesPerChat(ctx context.Context, createdAfter time.Time) ([]database.GetChatMessageSummariesPerChatRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatMessageSummariesPerChat(ctx, createdAfter)
+	m.queryLatencies.WithLabelValues("GetChatMessageSummariesPerChat").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatMessageSummariesPerChat").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatMessagesByChatID(ctx, chatID)
@@ -1224,6 +1232,14 @@ func (m queryMetricsStore) GetChatModelConfigs(ctx context.Context) ([]database.
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatModelConfigsForTelemetry(ctx context.Context) ([]database.GetChatModelConfigsForTelemetryRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatModelConfigsForTelemetry(ctx)
+	m.queryLatencies.WithLabelValues("GetChatModelConfigsForTelemetry").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatModelConfigsForTelemetry").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatProviderByID(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatProviderByID(ctx, id)
@@ -1336,6 +1352,14 @@ func (m queryMetricsStore) GetChatsByWorkspaceIDs(ctx context.Context, ids []uui
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatsUpdatedAfter(ctx context.Context, updatedAfter time.Time) ([]database.GetChatsUpdatedAfterRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatsUpdatedAfter(ctx, updatedAfter)
+	m.queryLatencies.WithLabelValues("GetChatsUpdatedAfter").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatsUpdatedAfter").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetConnectionLogsOffset(ctx context.Context, arg database.GetConnectionLogsOffsetParams) ([]database.GetConnectionLogsOffsetRow, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetConnectionLogsOffset(ctx, arg)