fix: support partial edits for files in CreateCommit tool

What does this merge request do and why?

Adds support for partial in-place file edits (old_str/new_str) to the CreateCommit tool, enabling fast and reliable updates to large files in agentic chat.

This avoids previous performance issues and timeouts caused by generating full file content for every change.

Relates to: gitlab-org/gitlab#572444 (closed)

How to set up and validate locally

1. Create a large file in your local repo (e.g., copy the contents of instance_limits.md).
2. In Agentic Chat, ask to update a single line in the file.
3. The change should be applied quickly, with no performance issues or timeouts.

rge 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.