fix: misleading tool description for context_addFiles

[CamilleLetavernier]

fixes #16777

What it does

• Clarify the description of the "addFiles" parameter in "context_addFile" tool, to make it consistent with the tool description and the actual tool implementation
  • Only paths relative to the workspace root are expected (e.g. packages/ai-ide/src/browser/context-functions.ts rather than /packages/ai-ide/src/browser/context-functions.ts)

How to test

• Hard to test, as LLMs are non-deterministic

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[ndoschek]

LGTM 👍