test(bigframes): Disable gcsfs cache for tests

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds a session-scoped pytest fixture to disable the gcsfs listings cache, preventing failures caused by stale file metadata during system tests. The review feedback suggests that simply instantiating GCSFileSystem is insufficient for global effect; instead, the configuration should be set via fsspec.config.conf and the instance cache should be cleared, requiring an additional import of fsspec.

[gemini-code-assist]

Instantiating gcsfs.GCSFileSystem(use_listings_cache=False) only creates a single instance with the cache disabled and does not affect other instances created elsewhere in the codebase (e.g., by bigframes internals or pandas). To globally disable the listings cache for all gcsfs instances during the test session, you should set the configuration in fsspec.config.conf and clear the instance cache to ensure any existing instances are recreated with the new settings.

Suggested change

	gcsfs.GCSFileSystem(use_listings_cache=False)
	fsspec.config.conf["gcs"] = {"use_listings_cache": False}
	gcsfs.GCSFileSystem.clear_instance_cache()

[gemini-code-assist]

Add fsspec to the imports. This is required to globally configure gcsfs settings (such as disabling the listings cache) for the entire test session via the fsspec configuration system.

Suggested change

	import gcsfs
	import fsspec
	import gcsfs