fix(bigquery): Fix bigquery socket leak

[chalmerlowe]

This Pull Request addresses flakiness in the socket leak system test and improves resource cleanup in the Database API (DB-API) connection.

Problem

1. The test test_dbapi_connection_does_not_leak_sockets was flaky in Continuous Integration (CI) environments because it relied on the psutil library to count Operating System sockets. Socket pooling, delayed garbage collection, and background noise make counting Operating System sockets non-deterministic.
2. Closing a database connection could leave underlying resources hanging if cursors were not closed in the correct order or if clients were not fully initialized.

Solution

1. Deterministic Testing: Rewrote the test to use Whitebox Object Tracking. Instead of counting Operating System sockets, the test intercepts the creation and closure of requests.Session and grpc.Channel objects using instance-level monkey patching. This ensures that every session or channel opened by the connection is explicitly accounted for and closed, independent of Operating System quirks.
2. Improved Cleanup: Updated Connection.close() in connection.py to close all created cursors before closing the clients themselves. This ensures that cursors release their references to query data and transports first. Added defensive checks to prevent errors if clients are None.

Note

• This test no longer uses psutil or artificial sleeps, making it faster and fully deterministic.
• The use of sets to track objects avoids false positives from double closures.
• Instance-level patching is used to isolate tracking to only the objects created within the test block.

[gemini-code-assist]

Code Review

This pull request refactors the test_dbapi_connection_does_not_leak_sockets system test to use a more deterministic approach by patching and tracking HTTP sessions and gRPC channels instead of relying on flaky psutil socket counts. The review feedback suggests using sets instead of lists to track these objects, which prevents potential flakiness from duplicate close() calls and simplifies both the assertions and the cleanup logic.