fix: retry post-creation API calls on 422 to handle GitHub eventual consistency

[dbeilin]

Summary

• Adds targeted retry logic for API calls made immediately after PR creation (labels, milestone, assignees, reviewers)
• Retries only 422 errors, only when the PR was just created (pull.created === true), up to 2 retries with 1s delay
• Does not change the global @octokit/plugin-retry configuration — this is scoped retry in the specific code path affected by GitHub API eventual consistency

Context

After creating a PR, follow-up REST API calls can fail with 422 Validation Failed: "Could not resolve to a node with the global id of '...'" because GitHub's API hasn't fully propagated the new PR internally. This is intermittent and timing-dependent.

Fixes #4321

Test plan

• New unit tests for retryWithBackoff utility (5 tests)
• All existing unit tests pass (17 total)
• All integration tests pass (50 total)
• npm run build succeeds

🤖 Generated with Claude Code

[dbeilin]

hi @peter-evans , this PR should address the label application issue.
It was 100% written by opus 4.6, hope that's not against your policy. Just thought I'd try, if you'd rather avoid implementing AI generated code, I totally understand.

Let me know if you have any concerns. We're using your Action in a lot of our workflows and have been experiencing that label apply issue pretty often.

Thanks