Fix streaming implementation issues

- Add streaming parameters to BaseAgent.execute_task method signature
- Fix mock LLM objects to include supports_stop_words attribute
- Update event emission to use crewai_event_bus.emit instead of direct method calls
- Remove unused variables in test files

Co-Authored-By: João <joao@crewai.com>

src/crewai/agents/agent_builder/base_agent.py

@@ -254,6 +254,8 @@ class BaseAgent(ABC, BaseModel):
         task: Any,
         context: Optional[str] = None,
         tools: Optional[List[BaseTool]] = None,
+        stream: bool = False,
+        stream_callback: Optional[Callable[[str, str, str, str], None]] = None,
     ) -> str:
         pass