fix(bigquery): [WIP] add gc.collect before socket leak test to reduce flakiness

[chalmerlowe]

Problem

The system test test_dbapi_connection_does_not_leak_sockets can be flaky, failing with assertions that more sockets are open at the end than at the start. This is often caused by non-deterministic garbage collection of sockets from previous tests running in the same process, which are finally collected during the test execution, or background noise that wasn't cleaned up before the test started.

Solution

Added gc.collect() explicitly at the very beginning of test_dbapi_connection_does_not_leak_sockets, before taking the initial socket count (conn_start). This ensures a cleaner baseline and reduces false positives caused by delayed cleanup from other tests.

Notes to Reviewers

This is a standard pattern for stabilizing process-wide socket count assertions in noisy test environments (like parallel execution with pytest-xdist). It complements the previous fixes that ensured explicit closure of channels and releasing of references.

[gemini-code-assist]

Code Review

This pull request adds a garbage collection call (gc.collect()) at the start of the test_dbapi_connection_does_not_leak_sockets test to clean up resources from previous tests and prevent false positives during socket leak detection. There are no review comments, and I have no additional feedback to provide.