ci: support release candidate publishing and PEP 440 release validation

[jplbrun]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Hardens and documents the existing support for publishing release candidates (RCs) to PyPI, and updates the version-bump automation to understand PEP 440 pre-release versions.

Previously the version-bump check used sort -V, which incorrectly treats 1.0.0rc1 as newer than the final 1.0.0. When the check ran for a source-changing PR, it could therefore misclassify an RC-to-final promotion as a regression. The release workflow also did not validate the release tag or the GitHub pre-release setting.

This PR:

• Replaces the shell sort -V comparison in check-version-bump.yaml with a Python script (check_version_bump.py) that uses PEP 440 ordering, so 1.0.0rc1 -> 1.0.0 is accepted and regressions are still rejected.
• Adds a validation step to release.yml (validate_release.py) that fails the publish before build if any of these are wrong: the version is not normalized PEP 440, the tag does not equal v<version>, or the GitHub pre-release setting does not match whether the version is a pre-release.
• Reads the current pre-release setting via gh api on each run, so a maintainer can correct a pre-release flag mismatch and re-run the failed workflow without creating a new release. (A tag mismatch still requires publishing a new release with the correct tag.)
• Adds unit tests for both scripts under tests/ci/.
• Updates docs/RELEASE.md with the RC cycle, the pre-release/latest guidance, and RC install instructions.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Run the script unit tests: uv run pytest tests/ci/test_versioning_scripts.py -v.
2. Confirm the version-bump check accepts 1.0.0rc1 -> 1.0.0 and rejects 1.0.0 -> 1.0.0rc1 (regression).
3. Confirm validate_release.py rejects a tag/version mismatch and a pre-release-flag mismatch, and accepts matching metadata for both an RC and a stable version.

Expected result: the RC-to-final bump is accepted, mismatched release metadata fails before publishing, and correct metadata passes.

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

None

Additional Notes

N/A