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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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>