fix(bigquery): close GAPIC storage transport and auth sessions to prevent socket leaks

[chalmerlowe]

This PR resolves resource leaks (specifically open sockets left in the ESTABLISHED state) that occur during client lifecycle operations and credential refreshing in system/unit tests.

The Problem

1. Transport Lifecycle: When closing the BigQuery Storage client, calling _transport.grpc_channel.close() was insufficient for releasing all network resources. The full _transport.close() method needs to be invoked to tear down the underlying transport channel correctly.

2. Dynamic Auth Sessions: Under certain authentication environments (like Workload Identity/GCE Metadata server inside Kokoro CI), the google-auth library dynamically instantiates helper HTTP sessions to fetch access tokens. These sessions are not owned by the BigQuery client and are not closed automatically, leading to leaked sockets.

3. Flaky Test Assertions: Socket count assertions in system tests were flaky because Python's garbage collection is non-deterministic, meaning sockets remained open in the operating system even after client close calls until a garbage collection cycle swept them.

Changes

• Client & Transport Lifecycle:

  • Updated connection.py, magics.py, and table.py to close the BigQuery Storage transport using _transport.close() instead of _transport.grpc_channel.close().

• Testing Improvements:

  • Added patch_tracked_requests interceptor to system/unit tests to track and explicitly close all dynamically spawned credential-refreshing HTTP sessions when the test context exits.
  • Added explicit gc.collect() calls to socket leak verification tests to force synchronous sweeping of unreferenced socket objects before asserting final socket counts.

• Code Coverage:

  • Appended # pragma: NO COVER to Python version checks in __init__.py for code paths that render a deprecation warning code if attempted to run on Python runtimes <3.10 test matrix (these paths do not run in our CI/CD since we never execute code with the older runtimes).

[gemini-code-assist]

Code Review

This pull request updates the BigQuery client to close the transport directly via _transport.close() instead of _transport.grpc_channel.close(), updates corresponding unit and system tests, and adds a new TestPropertyGraphReference test class. The review feedback suggests improving the patch_tracked_requests context manager in system tests to avoid closing pre-existing sessions, and refactoring manual debug prints to use idiomatic assertion messages instead.