impl: add gRPC PQC test

[scotthart]

Verifies MLKEM algorithms are being used with gRPC transport.

[gemini-code-assist]

Code Review

This pull request adds a new gRPC integration test to verify Post-Quantum Cryptography (PQC) using the X25519MLKEM768 group, alongside updates to the Bazel build configuration and the showcase test runner. It also refactors existing assertions in the REST PQC test to use modern GoogleTest/GoogleMock matchers. The review feedback highlights a potential undefined behavior issue in the new gRPC test when constructing std::string from potentially empty grpc::string_ref metadata keys or values, and suggests a safe extraction helper.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (a72ba49) to head (9c4d070).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16282   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files        2223     2223           
  Lines      207876   207876           
=======================================
+ Hits       191876   191883    +7     
+ Misses      16000    15993    -7     

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[scotthart]

/gcbrun