fix(ci): make version-bump check against merge-base

[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

The Check Version Bump workflow diffs github.event.pull_request.base.sha against head.sha to decide whether a version bump is required. That base SHA is the commit main pointed at when the PR was last synced, and it drifts behind main as other PRs merge. As a result the diff includes src/**/*.py files from already-merged PRs that the current branch never touched, so the check wrongly demands a version bump and a documentation-only PR ends up failing because unrelated source files appear in the comparison.

This changes the check to diff against the merge-base of the base and head commits (the point where the branch actually diverged from main) instead of the raw base ref.

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

Reproduced locally using the workflow's own logic against a drifted PR's real SHAs:

1. Old behavior (raw base.sha): source files from already-merged PRs appear in the diff → check FAILS demanding a version bump.
2. New behavior (merge-base): only the branch's own files appear → docs-only diff prints No source file changes under src/. Version bump not required. → check PASSES.
3. Negative case preserved: a diff that genuinely contains a src/**/*.py change without a version bump still matches the regex and FAILS with the existing error — enforcement is unchanged for real source changes.

Also tested end-to-end : applying the fix to docs-only PR branch (#329) flipped Check Version Bump from failing to passing.

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

None

[jeanscherf]

Great catch!