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/dbmetrics/querymetrics.go

@@ -1200,6 +1200,14 @@ func (m queryMetricsStore) GetChatDiffStatusByChatID(ctx context.Context, chatID
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatDiffStatusSummary(ctx context.Context) (database.GetChatDiffStatusSummaryRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatDiffStatusSummary(ctx)
+	m.queryLatencies.WithLabelValues("GetChatDiffStatusSummary").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatDiffStatusSummary").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatDiffStatusesByChatIDs(ctx context.Context, chatIDs []uuid.UUID) ([]database.ChatDiffStatus, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatDiffStatusesByChatIDs(ctx, chatIDs)