agent: Fix tool use in Gemini (#30689)

Thread doesn't run pending tools when `stop_reason` is not `ToolUse`.
Perhaps we should change that so that it always runs pending tools if
there are some, but for now this change just fixes setting `stop_reason`
for Google models.

Release Notes:

- N/A

crates/language_models/src/provider/google.rs

@@ -628,6 +628,7 @@ impl GoogleEventMapper {
         // responds with `finish_reason: STOP`
         if wants_to_use_tool {
             self.stop_reason = StopReason::ToolUse;
+            events.push(Ok(LanguageModelCompletionEvent::Stop(StopReason::ToolUse)));
         }
         events
     }