feat(coderd): add PR status summary to telemetry snapshots (#24379)

Adds aggregate PR counts (total, open, merged, closed) from
`chat_diff_statuses` to telemetry snapshots, giving visibility into AI
agent PR outcomes across deployments.

The existing telemetry system reports `Chats`, `ChatMessageSummaries`,
and `ChatModelConfigs`, but had no PR-level data. This adds a
`ChatDiffStatusSummary` field to the `Snapshot` struct with four
all-time counts derived from a single aggregate query.

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

- New SQL query `GetChatDiffStatusSummary` counts `chat_diff_statuses`
rows with non-NULL `pull_request_state`, grouped by state
(open/merged/closed).
- `ChatDiffStatusSummary` struct added to telemetry `Snapshot`,
collected via a parallel `eg.Go()` block in `createSnapshot()`.
- `dbauthz` wrapper uses `rbac.ResourceSystem` (telemetry-only pattern).
- Test covers both empty state (zero counts) and populated state (mixed
states + NULL-state exclusion).

</details>

> 🤖 Generated by Coder Agents

coderd/database/queries/chats.sql

@@ -934,6 +934,28 @@ SET
 WHERE
     chat_id = @chat_id::uuid;
 
+-- name: GetChatDiffStatusSummary :one
+-- Returns aggregate PR counts across all agent chats for telemetry.
+-- Deduplicates by PR URL so forked chats referencing the same pull
+-- request are counted once (using the most recently refreshed state).
+-- Total is derived from the three recognized state buckets and
+-- always equals open + merged + closed; other non-NULL states are
+-- intentionally excluded from these aggregates.
+WITH deduped AS (
+    SELECT DISTINCT ON (COALESCE(NULLIF(cds.url, ''), c.id::text))
+        cds.pull_request_state
+    FROM chat_diff_statuses cds
+    JOIN chats c ON c.id = cds.chat_id
+    WHERE cds.pull_request_state IN ('open', 'merged', 'closed')
+    ORDER BY COALESCE(NULLIF(cds.url, ''), c.id::text), cds.updated_at DESC, c.id DESC
+)
+SELECT
+    COUNT(*)::bigint AS total,
+    COUNT(*) FILTER (WHERE pull_request_state = 'open')::bigint AS open,
+    COUNT(*) FILTER (WHERE pull_request_state = 'merged')::bigint AS merged,
+    COUNT(*) FILTER (WHERE pull_request_state = 'closed')::bigint AS closed
+FROM deduped;
+
 -- name: GetChatCostSummary :one
 -- Aggregate cost summary for a single user within a date range.
 -- Only counts assistant-role messages.
@@ -1298,12 +1320,14 @@ WHERE chats.id = deletable.id
 -- 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, client_type
-FROM chats
-WHERE updated_at > @updated_after;
+    c.id, c.owner_id, c.created_at, c.updated_at, c.status,
+    (c.parent_chat_id IS NOT NULL)::bool AS has_parent,
+    c.root_chat_id, c.workspace_id,
+    c.mode, c.archived, c.last_model_config_id, c.client_type,
+    cds.pull_request_state
+FROM chats c
+LEFT JOIN chat_diff_statuses cds ON cds.chat_id = c.id
+WHERE c.updated_at > @updated_after;
 
 -- name: GetChatMessageSummariesPerChat :many
 -- Aggregates message-level metrics per chat for messages created