fix: retry addIssueLabels on 422 Issue not found

[marcopeise]

Summary

When release-please creates a release PR and immediately calls issues.addLabels, GitHub can return HTTP 422 Issue not found because the PR is not yet indexed on the replica serving label mutations (eventual consistency).

This retries addIssueLabels up to 3 times with linear backoff (1s, 2s, 3s) when that specific error occurs.

Production impact (reporter)

Reporter confirmed in a private production workflow (Apr 13, 2026) with issues: write configured. PR was created successfully; label step failed in the same second:

✔ Successfully opened pull request: 741
##[error]release-please failed: Validation Failed: {"resource":"Label","code":"unprocessable","field":"data","message":"Issue not found"} - https://docs.github.com/rest/issues/labels#add-labels-to-an-issue

Release PR was merged without autorelease: pending; no tag/release until manual recovery.

Test plan

• isIssueNotFoundLabelError unit tests for 422 detection
• nock integration tests: retry then success; exhausted retries throw GitHubAPIError

Fixes #2754

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