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/queries/chats.sql

@@ -1244,3 +1244,46 @@ DELETE FROM chats
 USING deletable
 WHERE chats.id = deletable.id
   AND chats.archived = true;
+
+-- name: GetChatsUpdatedAfter :many
+-- 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.
+SELECT
+    id, owner_id, created_at, updated_at, status,
+    (parent_chat_id IS NOT NULL)::bool AS has_parent,
+    root_chat_id, workspace_id,
+    mode, archived, last_model_config_id
+FROM chats
+WHERE updated_at > @updated_after;
+
+-- name: GetChatMessageSummariesPerChat :many
+-- 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.
+SELECT
+    cm.chat_id,
+    COUNT(*)::bigint AS message_count,
+    COUNT(*) FILTER (WHERE cm.role = 'user')::bigint AS user_message_count,
+    COUNT(*) FILTER (WHERE cm.role = 'assistant')::bigint AS assistant_message_count,
+    COUNT(*) FILTER (WHERE cm.role = 'tool')::bigint AS tool_message_count,
+    COUNT(*) FILTER (WHERE cm.role = 'system')::bigint AS system_message_count,
+    COALESCE(SUM(cm.input_tokens), 0)::bigint AS total_input_tokens,
+    COALESCE(SUM(cm.output_tokens), 0)::bigint AS total_output_tokens,
+    COALESCE(SUM(cm.reasoning_tokens), 0)::bigint AS total_reasoning_tokens,
+    COALESCE(SUM(cm.cache_creation_tokens), 0)::bigint AS total_cache_creation_tokens,
+    COALESCE(SUM(cm.cache_read_tokens), 0)::bigint AS total_cache_read_tokens,
+    COALESCE(SUM(cm.total_cost_micros), 0)::bigint AS total_cost_micros,
+    COALESCE(SUM(cm.runtime_ms), 0)::bigint AS total_runtime_ms,
+    COUNT(DISTINCT cm.model_config_id)::bigint AS distinct_model_count,
+    COUNT(*) FILTER (WHERE cm.compressed)::bigint AS compressed_message_count
+FROM chat_messages cm
+WHERE cm.created_at > @created_after
+  AND cm.deleted = false
+GROUP BY cm.chat_id;
+
+-- name: GetChatModelConfigsForTelemetry :many
+-- Returns all model configurations for telemetry snapshot collection.
+SELECT id, provider, model, context_limit, enabled, is_default
+FROM chat_model_configs
+WHERE deleted = false;