feat(bigquery): share one BigQuery Storage client per Client

[ohmayr]

Shares one BigQueryReadClient per Client for Arrow results, instead of creating a new one for every read and never closing it.

• Creates it on first use, with the client's credentials and universe domain, and closes it in Client.close().
• Uses a bqstorage_client passed by the caller instead, without closing it.
• Sets gRPC keepalive on its channel (grpc.keepalive_time_ms: 60000, grpc.keepalive_timeout_ms: 60000).
• Adds unit tests and system tests for sharing, closing, and cancelling one read early.

[gemini-code-assist]

Code Review

This pull request introduces a shared, thread-safe BigQuery Storage client for ARROW query results, allowing multiple queries to reuse a single client instance. It also configures the shared client's gRPC transport with keepalive pings to prevent idle connections from being dropped, and ensures the shared client is closed when the main BigQuery client is closed. The feedback suggests defensively handling the options parameter in create_channel to prevent a potential TypeError if None is passed.

[gemini-code-assist]

If options is explicitly passed as None to create_channel, calling list(options) will raise a TypeError: 'NoneType' object is not iterable. Using options or [] ensures defensive handling and prevents potential runtime crashes.

Suggested change

	def create_channel(*args, options=(), **channel_kwargs):
	options = list(options) + [
	("grpc.keepalive_time_ms", _BQSTORAGE_KEEPALIVE_TIME_MS)
	]
	return transport_class.create_channel(*args, options=options, **channel_kwargs)
	def create_channel(*args, options=None, **channel_kwargs):
	options = list(options or []) + [
	("grpc.keepalive_time_ms", _BQSTORAGE_KEEPALIVE_TIME_MS)
	]
	return transport_class.create_channel(*args, options=options, **channel_kwargs)

References

1. When processing optional parameters with a helper function, prefer placing the null or None check within the function body rather than at the call site to simplify the calling code and improve encapsulation.