fix: address PR comment #38 - remove duplicate User-Agent header

Resolves review thread PRRT_kwDOO6Hdxs5uu7z7
- Removed redundant User-Agent header from per-request headers in make_graphql_request()
- User-Agent is already set as default header on the shared HTTP client
- httpx merges per-request headers with client defaults, so client-level default is sufficient

unraid_mcp/core/client.py

@@ -193,7 +193,6 @@ async def make_graphql_request(
     headers = {
         "Content-Type": "application/json",
         "X-API-Key": UNRAID_API_KEY,
-        "User-Agent": f"UnraidMCPServer/{VERSION}",  # Custom user-agent
     }
 
     payload: dict[str, Any] = {"query": query}