fix: handle pre-existing releases and protected branch pushes in CI [3.x]

[ManukMinasyan]

Summary

• release.yml: Skip gh release create if the release already exists (prevents HTTP 422 when release is created manually before CI runs)
• changelog.yml: Use peter-evans/create-pull-request instead of git-auto-commit-action to work with protected branches that require PRs

Context

Both failures happened during the v3.1.0 and 2.3.6 releases:

1. Release workflow failed because the GitHub release was already created manually
2. Changelog workflow failed on 2.x because the branch is protected and rejects direct pushes

[Copilot]

Pull request overview

Updates the release automation workflows to be resilient to two real-world CI failure modes: pre-existing GitHub releases (created manually before CI runs) and protected branches that reject direct pushes.

Changes:

• Add a pre-check to skip gh release create when a release for the tag already exists.
• Replace direct-commit changelog updates with a PR-based flow via peter-evans/create-pull-request.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/release.yml	Checks for an existing release by tag and conditionally skips release creation to avoid HTTP 422.
.github/workflows/changelog.yml	Creates a PR for CHANGELOG updates (instead of pushing directly) and adds required PR write permissions.