Move tests to aws-smithy-mocks

[simonmarty]

Issue #, if available:

Description of changes: Accidentally closed #112 on a force push.

Move unit tests to aws-smithy-mocks for easier test coverage.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 97.55556% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.53%. Comparing base (219cb8e) to head (6fdf9bb).

Files with missing lines	Patch %	Lines
aws_secretsmanager_caching/src/lib.rs	97.55%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   92.30%   92.53%   +0.22%     
==========================================
  Files          15       15              
  Lines        2626     2826     +200     
  Branches     2626     2826     +200     
==========================================
+ Hits         2424     2615     +191     
- Misses        152      160       +8     
- Partials       50       51       +1     

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[reyhankoyun]

The is_current tests (test_get_cache_is_current_fast_refreshes, test_is_current_version_id_succeeds, test_is_current_version_stage_succeeds, test_is_current_both_version_id_and_version_stage_succeed) share ~90% identical setup: same GSV mock, same describe mock, same client construction, same assertion loop. A helper that takes optional version_id/version_stage and returns a configured client + mock rules would cut a lot of duplication. The old code had fake_client() for this, looks like we removed it for mock-based tests but didn't replace it.

[simonmarty]

Refactored using an LLM in 98f9306, still proofreading the work. Found a miss in 6fdf9bb