Add input validation to /chat/query endpoint

[Copilot]

The /chat/query endpoint was passing unvalidated input directly to the LLM, causing 500 errors when clients sent empty payloads or non-string values.

Changes

• Added validation to ensure query is a non-empty string before invoking the LLM chain
• Returns 400 with clear error message for invalid input

@app.post("/chat/query")
async def chat_query(req: Request):
    # ... chatbot initialization ...
    body = await req.json()
    query = body.get("query")
    if not isinstance(query, str) or not query.strip():
        raise HTTPException(status_code=400, detail="Query must be a non-empty string.")
    result = chain.invoke({"messages": [{"role": "user", "content": query}]})
    # ...

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