ci(workspace): add strict clippy checks to selected handwritten crates

[suzmue]

Add a strict clippy check to the GCB lint.sh and lint-unstable.sh scripts that runs with --no-deps on a targeted set of handwritten client libraries in non-test mode.

The strict clippy run checks for missing_docs and clippy::exhaustive_enums on: auth, bigquery, bigtable, datastore, firestore, gax, lro, pubsub, storage, and wkt.

To prepare the target crates for these checks, this commit:

• Documents the src/auth/build.rs build script to resolve missing_docs.
• Adds #[allow(clippy::exhaustive_enums)] to stable public and internal enums across the target crates to avoid public API breaking changes.

[gemini-code-assist]

Code Review

This pull request introduces stricter linting for handwritten crates by adding checks for missing documentation and non-exhaustive enums in the CI scripts. To accommodate these changes, documentation was added to the auth build script, and several enums across the workspace were annotated with #[allow(clippy::exhaustive_enums)]. The reviewer recommends adding the --all-features flag to the new clippy commands in the GCB scripts to ensure that public items and enums located behind feature gates are also validated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (02edf0b) to head (ade8812).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5635   +/-   ##
=======================================
  Coverage   97.76%   97.76%           
=======================================
  Files         221      221           
  Lines       51115    51115           
=======================================
+ Hits        49971    49972    +1     
+ Misses       1144     1143    -1     

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