feat: add integration test foundation with basic secret retrieval tests (#125)

*Issue #, if available:*

*Description of changes:* Added the foundation for AWS Secrets Manager integration tests with secret retrieval validation. This enables testing real AWS interactions that unit tests cannot cover, ensuring the agent works correctly with live AWS services.

*Changes*
* Integration test foundation (`tests/common/`) - Shared utilities for agent lifecycle management and AWS interactions
* Secret retrieval tests (`tests/secret_retrieval.rs`) - 6 tests covering basic AWS Secrets Manager features
* Local test runner (`test-local.sh`) - Automated execution of integration tests

*Testing*
Run locally with AWS credentials:

```
# Ensure AWS credentials are configured
ada credentials update --account=YOUR_ACCOUNT_ID --role=YOUR_ROLE

# Run all integration tests locally
./test-local.sh
```

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

Author: reyhankoyun