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/dbmock/dbmock.go

@@ -2207,6 +2207,21 @@ func (mr *MockStoreMockRecorder) GetChatDiffStatusByChatID(ctx, chatID any) *gom
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatDiffStatusByChatID", reflect.TypeOf((*MockStore)(nil).GetChatDiffStatusByChatID), ctx, chatID)
 }
 
+// GetChatDiffStatusSummary mocks base method.
+func (m *MockStore) GetChatDiffStatusSummary(ctx context.Context) (database.GetChatDiffStatusSummaryRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatDiffStatusSummary", ctx)
+	ret0, _ := ret[0].(database.GetChatDiffStatusSummaryRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatDiffStatusSummary indicates an expected call of GetChatDiffStatusSummary.
+func (mr *MockStoreMockRecorder) GetChatDiffStatusSummary(ctx any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatDiffStatusSummary", reflect.TypeOf((*MockStore)(nil).GetChatDiffStatusSummary), ctx)
+}
+
 // GetChatDiffStatusesByChatIDs mocks base method.
 func (m *MockStore) GetChatDiffStatusesByChatIDs(ctx context.Context, chatIds []uuid.UUID) ([]database.ChatDiffStatus, error) {
 	m.ctrl.T.Helper()