feat(auth): Add blocking Regional Access Boundary Lookup and Seed Support

[macastelaz]

In order for the gcloud CLI to support Regional Access Boundary, the Python auth SDK needs to support blocking lookups as well as allowing an initial seed RAB to be provided (gcloud will set this seed if the CLI has a locally cached valid RAB available).

Additional details can be found at go/rab-python-gcloud-one-pager

[gemini-code-assist]

Code Review

This pull request renames the "Trust Boundary" feature to "Regional Access Boundary" (RAB) and introduces a centralized management system for it. Key changes include the implementation of _RegionalAccessBoundaryManager for thread-safe state handling and background refreshes, and the update of multiple credential types to support this new mechanism. The with_trust_boundary method is deprecated in favor of with_regional_access_boundary. Review feedback recommends updating a type hint in the utility module to correctly reflect the expected data types and removing redundant error logging in the base credentials class to reduce log noise.

[gemini-code-assist]

This error log is redundant. All implementations of _build_regional_access_boundary_lookup_url() that can fail and return None already log a more specific error message. This generic log entry adds noise to the logs.

To avoid duplicate logging, please remove this _LOGGER.error call.

Suggested change

	if not url:
	raise exceptions.InvalidValue("Failed to build trust boundary lookup URL.")
	_LOGGER.error("Failed to build Regional Access Boundary lookup URL.")
	return None
	if not url:
	return None

References

1. Remove duplicate lines of code, especially duplicate assertions in tests, to keep the codebase clean and avoid redundancy.