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/dbmock/dbmock.go

@@ -2162,6 +2162,21 @@ func (mr *MockStoreMockRecorder) GetChatMessageByID(ctx, id any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessageByID", reflect.TypeOf((*MockStore)(nil).GetChatMessageByID), ctx, id)
 }
 
+// GetChatMessageSummariesPerChat mocks base method.
+func (m *MockStore) GetChatMessageSummariesPerChat(ctx context.Context, createdAfter time.Time) ([]database.GetChatMessageSummariesPerChatRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatMessageSummariesPerChat", ctx, createdAfter)
+	ret0, _ := ret[0].([]database.GetChatMessageSummariesPerChatRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatMessageSummariesPerChat indicates an expected call of GetChatMessageSummariesPerChat.
+func (mr *MockStoreMockRecorder) GetChatMessageSummariesPerChat(ctx, createdAfter any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessageSummariesPerChat", reflect.TypeOf((*MockStore)(nil).GetChatMessageSummariesPerChat), ctx, createdAfter)
+}
+
 // GetChatMessagesByChatID mocks base method.
 func (m *MockStore) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	m.ctrl.T.Helper()
@@ -2252,6 +2267,21 @@ func (mr *MockStoreMockRecorder) GetChatModelConfigs(ctx any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatModelConfigs", reflect.TypeOf((*MockStore)(nil).GetChatModelConfigs), ctx)
 }
 
+// GetChatModelConfigsForTelemetry mocks base method.
+func (m *MockStore) GetChatModelConfigsForTelemetry(ctx context.Context) ([]database.GetChatModelConfigsForTelemetryRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatModelConfigsForTelemetry", ctx)
+	ret0, _ := ret[0].([]database.GetChatModelConfigsForTelemetryRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatModelConfigsForTelemetry indicates an expected call of GetChatModelConfigsForTelemetry.
+func (mr *MockStoreMockRecorder) GetChatModelConfigsForTelemetry(ctx any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatModelConfigsForTelemetry", reflect.TypeOf((*MockStore)(nil).GetChatModelConfigsForTelemetry), ctx)
+}
+
 // GetChatProviderByID mocks base method.
 func (m *MockStore) GetChatProviderByID(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
 	m.ctrl.T.Helper()
@@ -2462,6 +2492,21 @@ func (mr *MockStoreMockRecorder) GetChatsByWorkspaceIDs(ctx, ids any) *gomock.Ca
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatsByWorkspaceIDs", reflect.TypeOf((*MockStore)(nil).GetChatsByWorkspaceIDs), ctx, ids)
 }
 
+// GetChatsUpdatedAfter mocks base method.
+func (m *MockStore) GetChatsUpdatedAfter(ctx context.Context, updatedAfter time.Time) ([]database.GetChatsUpdatedAfterRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatsUpdatedAfter", ctx, updatedAfter)
+	ret0, _ := ret[0].([]database.GetChatsUpdatedAfterRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatsUpdatedAfter indicates an expected call of GetChatsUpdatedAfter.
+func (mr *MockStoreMockRecorder) GetChatsUpdatedAfter(ctx, updatedAfter any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatsUpdatedAfter", reflect.TypeOf((*MockStore)(nil).GetChatsUpdatedAfter), ctx, updatedAfter)
+}
+
 // GetConnectionLogsOffset mocks base method.
 func (m *MockStore) GetConnectionLogsOffset(ctx context.Context, arg database.GetConnectionLogsOffsetParams) ([]database.GetConnectionLogsOffsetRow, error) {
 	m.ctrl.T.Helper()