[coryan]

Document additional design and review guidelines for hand-written crates and proc-macro crates in AGENTS.md, ARCHITECTURE.md, .gemini/styleguide.md, and
doc/contributor/howto-guide-handwritten-client-api.md:

• Give environment variables (such as *_EMULATOR_HOST, GOOGLE_CLOUD_UNIVERSE_DOMAIN, and GOOGLE_CLOUD_RUST_LOGGING) higher precedence than programmatic ClientBuilder options, and use anonymous credentials for emulators.
• Omit Eq and Hash on open protocol enums, preserve REST/ProtoJSON string unknown enum values via wkt::internal::UnknownEnumValue, keep opaque Unknown variants on protocol oneof unions, and retain all proto response fields.
• Clear sibling branches in mutually exclusive wire oneof setters and ensure all PartialEq fields on stub/model types have public accessors.
• Coordinate multi-layer retries, test outer async fn Send bounds with async move closures, enforce 100% __private proc-macro hygiene and pairwise flatten overlap checks, and test spans with TestLayer.

[gemini-code-assist]

Code Review

This pull request comprehensively updates the SDK's developer guidelines and architecture documentation (including AGENTS.md, ARCHITECTURE.md, and the handwritten client API guide) to establish strict rules for proc-macro hygiene, open protocol enums, environment variable precedence, multi-layer retry coordination, and async Send bounds. The feedback suggests correcting the pseudo-code examples in the documentation where unsupported inline string interpolation is used in Rust string literals.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (bb112d0) to head (89af070).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7462   +/-   ##
=======================================
  Coverage   98.60%   98.61%           
=======================================
  Files         338      338           
  Lines      148237   148237           
=======================================
+ Hits       146172   146180    +8     
+ Misses       2065     2057    -8     

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