fix: Fix chat freezing when messages contain special characters or network errors occur

[naaa760]

🐛 Problem

Users reported that the chat interface would freeze after submitting messages, especially with messages containing special characters (like Polish text). The UI would become unresponsive even though the backend was processing correctly.

#4841

Solution

Fixed EventSource error handling that was causing the chat to freeze:

• Removed error throwing** in EventSource onerror handler that was breaking the UI
• Added safe error property access** to prevent crashes when error objects don't have expected structure
• Enhanced connection validation** to handle failed EventSource connections gracefully
• Protected JSON parsing** to prevent single malformed messages from breaking the entire chat
• Added connection-level error handling** with user-friendly fallback messages

✅ Testing

• Chat no longer freezes with special character messages
• Error scenarios are handled gracefully with proper user feedback
• Build passes without errors
• Existing functionality preserved

Impact

• Better user experience with stable chat interface
• Improved error handling and debugging
• More resilient to network issues and server errors

[HenryHengZJ]

we have to throw error here otherwise fetchEventSource will keep retrying non stop causing unlimited calls to the LLM.

previously this had caught us surprised with LLM bills up to thousands due to not throwing the error

[HenryHengZJ]

thanks for the help! however I dont think that's the cause to the issue