fix: allows current sub-agent to finish execution before exiting the loop agent due to a sub-agent's escalation. This commit also disables the summarization for exit_loop tool

Fixes #423

Related to #1670

- This avoids the `GeneratorExit` error thrown, which would crash OTel metric collection and cause `Failed to detach context` error.
- This also allows all function  calls are processed when exit_loop is called together with other tools in the same LLmResponse.

A sample agent for testing:

```
from google.adk import Agent
from google.adk.agents.loop_agent import LoopAgent
from google.adk.tools.exit_loop_tool import exit_loop

worker_1 = Agent(
    name='worker_1',
    description='Worker 1',
    instruction="""\
Just say job #1 is done.

If job #1 is said to be done. Call exit_loop tool.""",
    tools=[exit_loop],
)

worker_2 = Agent(
    name='worker_2',
    description='Worker 2',
    instruction="""\
Just say job #2 is done.

If job #2 is said to be done. Call exit_loop tool.""",
    tools=[exit_loop],
)

work_agent = LoopAgent(
    name='work_agent',
    description='Do all work.',
    sub_agents=[worker_1, worker_2],
    max_iterations=5,
)

root_agent = Agent(
    model='gemini-2.0-flash',
    name='hello_world_agent',
    description='hello world agent that can roll a  check prime',
    instruction="""Hand off works to sub agents.""",
    sub_agents=[work_agent],
)
```
PiperOrigin-RevId: 785538101

tests/unittests/flows/llm_flows/test_agent_transfer.py

@@ -303,11 +303,9 @@ def test_auto_to_loop():
               name='exit_loop', response={'result': None}
           ),
       ),
-      # root_agent summarizes.
-      ('root_agent', 'response4'),
   ]
 
   # root_agent should still be the current agent because sub_agent_1 is loop.
   assert testing_utils.simplify_events(runner.run('test2')) == [
-      ('root_agent', 'response5'),
+      ('root_agent', 'response4'),
   ]