Enhance PR Listing and Checkout Logic

This ticket covers two improvements to the pullRequestService:

1. Filter by State: The listPullRequests function has been enhanced to allow filtering pull requests by their state (open, closed, merged, all). This is exposed via a state query parameter in the GET /pull-requests endpoint.

2. Improved Checkout Response: The checkoutPullRequest function was updated to include the stdout from the gh pr checkout command in its response. This provides more useful feedback to the caller (e.g., the name of the branch that was checked out).

Acceptance Criteria

1. The openapi.yaml is updated to include the optional state query parameter for the GET /pull-requests endpoint.
2. pullRequestService.mjs is updated:
   • listPullRequests now accepts a state option and passes it to the gh pr list --state command.
   • checkoutPullRequest now returns an object containing the details from the command's stdout.
3. The pullRequests.mjs route is updated to read the state from the query string and pass it to the service.