fix: retry post-creation API calls on 422 eventual consistency errors (#4356)

Add retry logic to handle GitHub API eventual consistency errors that
can occur after creating a new pull request. Follow-up API calls for
milestones, labels, assignees, and reviewers may fail with a 422
"Could not resolve to a node" error before the PR is fully propagated.

- Add generic `retryWithBackoff` helper in `src/utils.ts` with
  exponential backoff (default 2 retries, starting at 1s delay)
- Wrap post-creation API calls in `src/github-helper.ts` with
  `withRetryForNewPr()`, which only retries for newly created PRs
- Use `@octokit/request-error` `RequestError` type for precise error
  matching (status 422 + "Could not resolve to a node" message)
- Add unit tests for `retryWithBackoff` covering success, retry,
  exhaustion, and non-retryable error scenarios
- Update `dist/index.js` bundle and `package.json` dependencies

package.json

@@ -39,6 +39,7 @@
     "@octokit/plugin-rest-endpoint-methods": "^13.5.0",
     "@octokit/plugin-retry": "^7.2.1",
     "@octokit/plugin-throttling": "^9.6.1",
+    "@octokit/request-error": "^6.1.8",
     "node-fetch-native": "^1.6.7",
     "p-limit": "^6.2.0",
     "uuid": "^9.0.1"