Add optional runner availability gate using actions/python-versions manifest

[CasperKristiansson]

Summary

Introduce an opt-in require_runner flag that skips upgrades when the resolved patch version is not yet available on GitHub-hosted runners, citing the missing entry in actions/python-versions' versions-manifest.json.

Why

Teams depending on runners need assurance that the proposed patch can actually execute in CI. Today the action may open a PR for a version that is not live yet, leading to broken pipelines.

What needs to happen

• Fetch (or use cached snapshot) of the versions-manifest.json from actions/python-versions when the flag is enabled.
• Compare the candidate patch version against runner availability per OS.
• Surface a descriptive skip reason (and include details in the PR body when skipped).
• Document how to enable the check and its trade-offs.
• Add unit tests with a manifest fixture covering available and missing versions.

References

• Roadmap hardening item: "Runner availability gate".
• https://github.com/actions/python-versions/blob/main/versions-manifest.json

Acceptance criteria

• With require_runner: true, the action skips and reports when a patch is unavailable on hosted runners.
• Output or PR text mentions which runners are missing coverage.
• Disabled by default to preserve current behavior.