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

src/google/adk/agents/loop_agent.py

@@ -53,10 +53,15 @@ class LoopAgent(BaseAgent):
     times_looped = 0
     while not self.max_iterations or times_looped < self.max_iterations:
       for sub_agent in self.sub_agents:
+        should_exit = False
         async for event in sub_agent.run_async(ctx):
           yield event
           if event.actions.escalate:
-            return
+            should_exit = True
+
+        if should_exit:
+          return
+
       times_looped += 1
     return
 

src/google/adk/tools/exit_loop_tool.py

@@ -21,3 +21,4 @@ def exit_loop(tool_context: ToolContext):
   Call this function only when you are instructed to do so.
   """
   tool_context.actions.escalate = True
+  tool_context.actions.skip_summarization = True