fix: declare agent-specific variable in ClaudeCode agent and improve agent specific variable status UX

[ndoschek]

What it does

Resolves GH-16912

• fix(ai-claude-code): declare agent-specific variable
  • Add activeEditor to agentSpecificVariables so it no longer show as "Undeclared" in the AI Configuration view as it is effectively used in the prompt
• fix(ai-ide): improve agent specific variable status UX
  • Add tooltips to undeclared and not-used-in-prompt variable warnings
  • Fix warning text color to improve readability

How to test

• Check the "Used Agent-Specific Variables" section of the ClaudeCode agent in the AI Configuration View, activeEditor should now have a proper description
• To test the UX improvements for the warnings:
  1. Add a random undeclared variable to the ClaudeCode prompt, for example {{myUndeclaredVariable}}. Verify that the status "Undeclared" is displayed and styled correctly. Check the tooltip for more information.
  2. Remove {{activeEditor}} from the ClaudeCode prompt. Verify that the status "Not used in prompt" is displayed and styled correctly. Check the tooltip for more information.

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

[planger]

Thank you very much! Looks good.