fix: resolve runtime issues and improve validation & error handling

[chaitanya-codes]

This PR addresses several stability and reliability issues in the message handler:

• Fixed undefined variable reference in custom command handler
• Corrected chatbot validation logic to prevent empty or invalid responses
• Fixed incorrect message object usage in external API call
• Removed deprecated timeout option from sticky message deletion
• Improved error handling by replacing silent promise failures with logging
• Added defensive channel existence checks to prevent potential runtime crashes

These changes improve overall runtime stability, maintainability, and error visibility without altering existing functionality.