Fix draft PR creation for repositories that don't support them

[codegen-sh]

This PR addresses the issue where Codegen fails to create PRs in repositories that don't support draft PRs.

Changes

1. Modified the create_pull method in GitRepoClient to:

   • Try creating a draft PR first if requested
   • If it fails with the specific error "Draft pull requests are not supported in this repository", fall back to creating a regular PR
   • Improve error handling and logging

2. Added a new utility method accepts_draft_prs() to GitRepoClient that:

   • Provides a way to check if a repository supports draft PRs
   • Uses a heuristic based on repository visibility to avoid unnecessary API calls
   • Returns a boolean indicating whether draft PRs are supported

Testing

The implementation has been designed to:

• Work without making additional API calls when possible
• Gracefully handle the case where draft PRs are not supported
• Maintain backward compatibility with existing code

Fixes CG-11610