[ohmayr]

Makes BigQueryReadClient send gRPC keepalive pings by default. ReadRows streams can sit idle for a long time, for example while BigQuery finishes a query job, and without pings a NAT or load balancer can drop the idle connection without telling either side. google-cloud-bigquery works around this with its own transport for Arrow query results (#18652). This moves the setting here, so every long read gets it.

• Uses a gRPC channel with grpc.keepalive_time_ms and grpc.keepalive_timeout_ms set to 60s when no transport is passed (or transport="grpc").
• Leaves a caller's transport instance or callable unchanged.
• Adds unit tests for the default channel options and for caller-provided transports.

This changes the default channel for every bigquery-storage user, but normal table reads shouldn't notice: pings only go out while a call is open and the connection has been quiet for 60s (keepalive_permit_without_calls isn't set). A read on a dropped connection now fails with UNAVAILABLE after about 2 minutes instead of hanging, and ReadRowsStream already reconnects on that. google-cloud-bigtable and google-cloud-firestore already send keepalive pings by default, every 30s.

Fixes #18784

[gemini-code-assist]

Code Review

This pull request introduces default gRPC keepalive pings (60 seconds) for the BigQueryReadClient to prevent idle ReadRows streams from being dropped by NATs or load balancers, along with corresponding unit tests. Feedback on the changes highlights a potential issue where unconditionally appending default keepalive options could override custom user-configured channel options. It is recommended to check for existing keepalive keys in the options list before appending the defaults.

[tswast]

Thanks!