fix: checkout existing release branch in release workflow

[philbrookes]

Summary

• Fix release workflow failing when the release branch already exists on origin (e.g. patch releases like v0.25.2)
• Check for the remote branch first and fetch/checkout it if present, otherwise create a new one

Problem

The release workflow unconditionally ran git checkout -b <release-branch>, which created a new local branch from the checked-out gitRef. When the release branch already existed on origin (from a previous release), the subsequent git push was rejected because the local branch diverged from the remote.

See: https://github.com/Kuadrant/authorino-operator/actions/runs/29233186429/job/86761778828

Summary by CodeRabbit

• Bug Fixes
  • Improved release creation workflows by reliably checking out existing release branches or creating new ones when needed.
  • Increased consistency for subsequent release preparation and publishing steps.

[coderabbitai]

📝 Walkthrough

Walkthrough

The release workflow now checks out an existing remote release branch when available and creates a new branch otherwise, before continuing with release preparation and publication.

Changes

Release branch flow

Layer / File(s)	Summary
Conditional release branch selection .github/workflows/release.yaml	The workflow fetches and checks out an existing release branch from origin, or creates it when the branch does not exist.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Poem

I’m a rabbit with branches to spare,
Fetching old ones with care.
If none can be found,
A new branch is spun round—
Releases hop smoothly from there.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: the release workflow now checks out an existing release branch instead of always creating one.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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