docs: clarify postgres-mode None + test exact=True default (review #850)

- Note at both metrics-task call sites that receive_stream is None in postgres
  mode (get_ingest_pending falls back to procrastinate counts).
- Add test_default_is_exact_true covering the status-endpoint count path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_metrics_publisher.py

@@ -78,6 +78,15 @@ class TestCountIndexed:
 
         assert all(call.kwargs["exact"] is False for call in qc.count.await_args_list)
 
+    async def test_default_is_exact_true(self) -> None:
+        # The on-demand status endpoint relies on the exact=True default.
+        qc = AsyncMock()
+        qc.count.side_effect = [_count_obj(10), _count_obj(3)]
+
+        await mp.count_indexed(qc, _COLLECTION)
+
+        assert all(call.kwargs["exact"] is True for call in qc.count.await_args_list)
+
 
 class TestPublishVectorSyncMetrics:
     @pytest.fixture(autouse=True)