fix: return None for hidden stop tokens to prevent EOS in output (#5238)

Signed-off-by: Chaitanya Malakala <106723863+chay1045@users.noreply.github.com>

lib/llm/src/backend.rs

@@ -456,7 +456,7 @@ impl Decoder {
         // check for hidden stop tokens - eos takes precedence
         if self.hidden_stop_ids.contains(&token_id) {
             return Ok(StepResult::with_stop_trigger(
-                token,
+                None,
                 StopTrigger::HiddenStopTokenDetected(token_id),
             ));
         }