fix(chat): add input length validation to prevent payload abuse

[devprashant19]

Description

This PR resolves the missing input validation vulnerability on the LLM /chat endpoint. Previously, user input was passed directly to the Groq API without size restrictions, opening the application up to potential token exhaustion or billing spike attacks through excessively large payloads.

Changes

• Added a length check constraint on the message input in backend/routes/chatRoutes.js.
• If the payload exceeds 1,000 characters, the server now intercepts it and returns a 400 Bad Request before sending anything to Groq.

Related Issues

• Resolves: Missing Input Length Validation on LLM Chat Route #243

Type of Change

• Security fix
• Bug fix
• New feature
• Refactoring

Pre-submission Checklist

• Verified that messages under 1000 characters are processed normally.
• Verified that messages over 1000 characters return a clear 400 error.

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