fix: sanitize @OpenHands only in GitHub comments (backend)

[YakshithK]

Summary

This PR implements backend sanitization of @OpenHands mentions in GitHub comments to prevent self-mention loops. Instead of mentioning the OpenHands agent, the sanitization inserts a zero-width joiner after the @ symbol, making the mention non-clickable while maintaining readability.

Changes

• Added sanitize_openhands_mentions() function in openhands/sdk/utils/github.py that uses a zero-width joiner (U+200D) to prevent @OpenHands mentions from being parsed as clickable references
• Updated GitHub workflow files to sanitize @OpenHands mentions before posting comments
• Updated example agent script to sanitize review comments before posting
• Exported the new function from the utils module

Technical Details

• The sanitization is performed in the backend (when outputting to GitHub comments) rather than at the agent/frontend level
• Uses regex pattern matching to identify @OpenHands mentions (case-insensitive)
• Inserts zero-width joiner after @ to break the mention linkage
• Maintains text readability since the zero-width joiner is invisible

Related PR

See OpenHands/OpenHands#11550 for the corresponding changes in the main repository.