Sandcastle Copilot: Tool registry handling fix

[Guillar1]

#13614

Testing plan

Open Copilot → close it → edit the JS → reopen → ask for an edit. Before the fix every apply_diff fails with the nonsense closest-match hints; after it they apply normally.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

AI acknowledgment

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[github-actions]

Thank you for the pull request, @Guillar1!

✅ We can confirm we have a CLA on file for you.

[lukemckinstry]

Looks good, the fix works as described 👍