fix: merge multiple updates for same file in create_commit

What does this merge request do and why?

When create_commit receives multiple update actions for the same file, merge them into a single action by fetching the file once and sequentially applying all changes (content replacements and partial edits). Previously only the last action was applied.

How to set up and validate locally

1. Run Duo Agent platform and GDK.
2. Ask Duo to update some file with multiple different changes.
3. Ensure commit was successfully with all the requested changes.

Screen Recording 2025-12-04 at 15.06.33.mov

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.

Relates to: gitlab-org/gitlab#582447