impl(oauth2): add RAB integration testing

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces integration tests for Unified REST credentials, specifically covering Resource-Aware Binding (RAB) scenarios for Service Account (OAuth and JWT), Impersonated Service Account, and Compute Engine credentials. The feedback identifies a recurring safety issue across all new test cases where the code accesses the value of a StatusOr object without first verifying its status via headers.ok(), which could lead to undefined behavior upon RPC failure. The reviewer suggests updating the retry loops to include this safety check and to evaluate the condition before sleeping to avoid unnecessary delays.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.70%. Comparing base (cf931ff) to head (dc86d0d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16103   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files        2353     2353           
  Lines      218354   218437   +83     
=======================================
+ Hits       202419   202497   +78     
- Misses      15935    15940    +5     

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