Warning: This build has drifted.
The coverage report for this pull request build may be inaccurate because its base commit is no longer the HEAD of its target branch.
This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

    • Learn more: For more information on this, see Tracking coverage changes for pull request builds.
    • Fix now: For a quick fix, rebase this PR at GitHub. Your next report should be accurate.
    • Prevent going forward: To avoid this issue with future PRs, see these Recommended CI Configurations.

feat: Add credential fallback for fixture recording

Implements automatic fallback to existing fixtures when credentials are
missing during RECORD mode for providers requiring cloud credentials.

- Add optional `credential_missing?/1` callback to Provider behavior
- Implement credential detection in AWS Bedrock, Google, and Google Vertex AI providers
- Add fixture fallback handler in test fixture system
- Enables comprehensive test coverage without requiring all developers to configure cloud credentials

When recording fixtures (`REQ_LLM_FIXTURES_MODE=record`):
1. Provider attempts API call
2. If credentials missing, provider raises specific error
3. Credential fallback handler catches error via `credential_missing?/1`
4. If fixture exists, loads existing fixture instead of failing
5. Logs warning about credential fallback
6. Test continues successfully

This allows contributors without AWS/GCP credentials to run full test suite
while maintainers with credentials can refresh fixtures.