test(bigquery-magics): make table_id parsing check version-agnostic

The unit test test_bigquery_magic_query_variable_not_identifier was asserting that a specific error message ("must be a fully-qualified ID") was in the stderr output.

However, in a recent versions of dependencies, this error message changed (depending on whether tests were run under Python 3.10 or 3.11+). There is a key term that is common to both of the error messages: table_id.

This change confirms that we get one of the two expected error messages by checking for table_id, making it compatible regardless of which runtime the tests run under.

refactor(magics): replace fragile psutil socket check with client close mock spy

The system test test_bigquery_magic was using psutil to count open sockets before and after the test to detect leaks (i.e. ensure all the system resources were released).

This approach proved to be fragile across different environments (e.g., Kokoro).

This change replaces the socket counting with a mock spy on google.cloud.bigquery.Client.close to verify that the client is explicitly closed, which is the intended mechanism for releasing system resources.

test(bigquery-magics): simplify IPython initialization in deprecation test

The unit test test_cell_magic_engine_bigframes_warning in test_deprecation.py was conditionally initializing IPython. Only one of the code paths was being exercised so we were getting a coverage issue.

This change simplifies the initialization by unconditionally calling start_ipython() and assigning the returned instance to ip ensuring a clean and consistent state for the test and avoiding the need for a conditional check.