Fix flaky test tests/test_nodes_async.py::test_the_node_reads_open_no_sync_session_on_the_loop

The merge lane saw `tests/test_nodes_async.py::test_the_node_reads_open_no_sync_session_on_the_loop` fail and then pass on a rerun of the same tree. - Test: `tests/test_nodes_async.py::test_the_node_reads_open_no_sync_session_on_the_loop` - Tree: `1495159c6489b83c1d70de0ac1f193be6118d8c7` - Lane task: 13818 - Flake record: 01a1a647-bd84-79a9-a062-34e6fa9a39cc Make it deterministic; do not delete it unless it tests nothing.

Fixed the flaky test_nodes_async test. The watcher now counts only this test's own requests (a ContextVar), not lifespan or leftover tasks on the shared portal loop. Repro fails before the fix and passes after; a positive control still fails. Quick gate green. PR #7956 queued to the lane. Memory: none applied.