fix(tools): Add proper cleanup for AgentTool to prevent MCP session errors

Merge https://github.com/google/adk-python/pull/3411

## Summary

Fixes AgentTool cleanup to prevent MCP session errors by calling `runner.close()` after sub-agent execution.

## Problem

When using AgentTool with MCP tools, the runner cleanup happened during garbage collection in a different async task context, causing:
```
RuntimeError: Attempted to exit cancel scope in a different task than it was entered in
```

## Solution

- Call `await runner.close()` immediately after sub-agent execution in AgentTool
- This ensures MCP sessions and other resources are cleaned up in the correct async task context
- Updated test mock to include the close() method

## Demo Agents

Added two comprehensive demo agents showing how to use AgentTool with MCP tools:

### mcp_in_agent_tool_remote (SSE mode)
- Uses HTTP/SSE connection to remote MCP server
- Zero-installation setup with `uvx`
- Demonstrates server-side MCP deployment pattern

### mcp_in_agent_tool_stdio (stdio mode)
- Uses subprocess connection with automatic server launch
- Fully automatic setup with `uvx` subdirectory syntax
- Demonstrates embedded MCP deployment pattern

Both demos:
- Use Gemini 2.5 Flash
- Include example prompts for JSON Schema exploration
- Have comprehensive READMEs with architecture diagrams
- Follow ADK agent structure conventions

## Testing

- ✅ All existing unit tests pass
- ✅ Manual testing with both SSE and stdio modes
- ✅ Verified cleanup happens in correct async context
- ✅ No more cancel scope errors with MCP tools

## Related

- Fixes #1112
- Related to #929

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3411 from google:fix/agent-tool-mcp-cleanup 9ae753b5a4
PiperOrigin-RevId: 828651896

tests/unittests/tools/test_agent_tool.py

@@ -131,6 +131,10 @@ async def test_agent_tool_inherits_parent_app_name(monkeypatch):
       )
       return _empty_async_generator()
 
+    async def close(self):
+      """Mock close method."""
+      pass
+
   monkeypatch.setattr('google.adk.runners.Runner', StubRunner)
 
   tool_agent = Agent(