Fix GCP provider

[Calvin-Huang]

Pull Request

Description

The GCP provider was broken. It seems the multi-instance config feature wasn’t implemented in the GCP provider.

Also, the GCP_SA_JSON environment variable wasn’t properly loaded into the config. The current implementation only uses GCP_SERVICE_ACCOUNT_KEY as the credential provider, so I added support for loading credentials from a file path.

Type of Change

• Bug fix
• New feature
• Enhancement to existing functionality
• Documentation update

Testing

I followed the instructions in TESTING.md and also used a proxy locally.

Checklist

• I've run pytest locally and all tests pass
• I've added tests for new functionality (if applicable)
• My code follows the project's style
• I've updated documentation if needed

Important Note

All PRs must pass the automated test suite before they can be merged. The GitHub Actions workflow will automatically run pytest on your changes using the python-app-testing.yml workflow.