feat(spanner): Cleanup GcpFallbackChannel creation and enable by default

[kinsaurralde]

• Clean up setupGcpFallback by using new getDecoratedChannelBuilder instead of capturing cloudpath ChannelConfigurator
• Set EEF period to 3 minutes
• Enable fallback by default (still only used when direct access is enabled)

[gemini-code-assist]

Code Review

This pull request enables GCP fallback by default in SpannerOptions and refactors GapicSpannerRpc to simplify fallback channel setup using createDecoratedChannelBuilder. It also introduces a 3-minute period for fallback options. Feedback identifies a missing import for java.time.Duration and notes that the 10ms sleep in the integration tests is now insufficient to trigger the fallback check due to the increased period.

[gemini-code-assist]

The Duration class is used in this file (e.g., at line 566) but it is not imported. Please add the missing import for java.time.Duration.

Suggested change

	import java.util.concurrent.Future;
	import java.time.Duration;
	import java.util.concurrent.Future;

[gemini-code-assist]

The test waits for 10ms, but the production code now sets the fallback period to 3 minutes (Duration.ofMinutes(3)). This will likely cause the test to fail as the fallback check won't trigger within the sleep duration. Consider overriding the period in the test to a smaller value or adjusting the test expectations. Additionally, please remove the stale code comment '10ms period' instead of updating it, as it no longer reflects the implementation.

References

1. Remove comments that refer to stale implementations instead of updating them.

[gemini-code-assist]

Similar to the previous test case, the 10ms sleep is likely insufficient given the new 3-minute fallback period set in GapicSpannerRpc. The test should be updated to ensure the fallback check is triggered.