test: add GIL release verification tests

[beinan]

Summary

Adds comprehensive tests to verify that the Python Global Interpreter Lock (GIL) is properly released during graph operations, addressing issue #97.

Findings

✅ The codebase already properly releases the GIL!

The existing BackgroundExecutor (introduced in python/src/executor.rs) correctly uses py.allow_threads() to release the GIL during all blocking operations:

• block_on() method releases GIL before blocking on futures
• spawn() method releases GIL when spawning background tasks
• All Python binding methods use RT.block_on(Some(py), ...) which properly releases the GIL

Tests Added

This PR adds verification tests to document and ensure this behavior continues:

1. test_gil_release_during_query_execution - Verifies GIL release during CypherQuery.execute()
2. test_gil_release_during_to_sql - Verifies GIL release during CypherQuery.to_sql()
3. test_gil_release_during_explain - Verifies GIL release during CypherQuery.explain()
4. test_concurrent_query_execution - Tests parallel query execution across multiple threads
5. test_gil_release_with_heavy_query (slow) - Tests GIL release with large datasets

Test Strategy

Each test:

1. Starts a background thread that increments a heartbeat counter
2. Executes a graph operation in the main thread
3. Verifies the background thread made progress (heartbeats > threshold)

If GIL was not released, the background thread would be starved and heartbeats would be 0.

Test Results

============================= test session starts ==============================
python/tests/test_gil_release.py::test_gil_release_during_query_execution PASSED
python/tests/test_gil_release.py::test_gil_release_during_to_sql PASSED
python/tests/test_gil_release.py::test_concurrent_query_execution PASSED
python/tests/test_gil_release.py::test_gil_release_during_explain PASSED
python/tests/test_gil_release.py::test_gil_release_with_heavy_query PASSED

============================== 5 passed in 0.45s

✅ All tests pass

Closes

Closes #97

🤖 Generated with Claude Code