fix(ffi): replace todo!() panic with Err in ResponseValue::from_value

The catch-all arm for unsupported value types (Push, BigInt, Attribute)
previously used todo!() which panics. Panicking across an FFI boundary
is undefined behavior. Return an error string instead.

Signed-off-by: currantw <taylor.curran@improving.com>

Author: currantw

rust/src/ffi.rs

@@ -734,7 +734,7 @@ impl ResponseValue {
                     size,
                 })
             }
-            _ => todo!(), // push, bigint, attribute
+            _ => Err("Unsupported Redis value type in FFI response serialization".into()),
         }
     }