Handle AI provider errors in WebSocket chat and clear loading state

[arefbehboudi]

This change improves WebSocket chat error handling when the AI provider request fails, so the socket is not closed by an uncaught exception and the chat UI does not remain stuck in a loading state.
What changed:

• Added defensive error handling in ChatWebSocketHandler.java
• Return an in-chat error bubble instead of failing silently
• Added and updated tests for the new error-handling behavior in ChatWebSocketHandlerTest.java

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Thanks a lot!

[arefbehboudi]

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