fix: allow reserved names in tool output mappings

[Vietpr]

Related Issues

• Fixes Tool outputs_to_string fails when multi-output tools use "source", "handler", or "raw_result" as output keys #13109

Proposed Changes

• Allow 'source', 'handler', and 'raw_result' as names in multi-output 'outputs_to_string' configurations.
• Add regression tests and a release note.

How did you test it?

• Unit tests passed.
• Pre-commit checks passed.

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[HaystackBot]

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Thanks for signing the CLA, @Vietpr! 🎉 This PR is now ready for review again and the reviewer has been re-assigned.