chore: replaced hard coded value for user_id to the value from the tool context from parent agent. Fixes google/adk-python#2407

Merge https://github.com/google/adk-python/pull/2409

Description:

This PR Fixes: #2407

The AgentTool in /google/adk/tools/agent_tool.py uses a hardcoded user_id='tmp_user' when creating a new session for the agent it wraps. This happens within the run_async method.

code snippet
... @override async def run_async( self, *, args: dict[str, Any], tool_context: ToolContext, ) -> Any: ... session = await runner.session_service.create_session( app_name=self.agent.name, user_id='tmp_user',  # <-- This is hardcoded state=tool_context.state.to_dict(), ) ...

Why is this a problem?
This hardcoding breaks the chain of user identity. When a parent agent calls a sub-agent via the AgentTool, the original user_id is lost. Any tool or logic inside the sub-agent that needs to perform user-specific actions (e.g., accessing user data from a database, retrieving user-specific memory, checking permissions) will fail or operate on the wrong context because it receives 'tmp_user' instead of the actual user's ID.

Impact:
This prevents the creation of robust, multi-agent applications where user context must be maintained across different agents and tools. It limits the utility of AgentTool to only stateless sub-agents that do not require user-specific information.

Suggested Fix:
The user_id should be retrieved from the parent context, which is available via the tool_context parameter passed into run_async. The create_session call should be updated to use the dynamic user_id from the parent session.For example, the fix might involve accessing the user ID from the tool_context.

code-snippet
session = await runner.session_service.create_session( app_name=self.agent.name, user_id=tool_context._invocation_context.user_id, state=tool_context.state.to_dict(), )

To Reproduce
Steps to reproduce the behavior:
To reproduce this bug, we need to set up a two-agent system: a ParentAgent that calls a ChildAgent using the AgentTool. The ChildAgent will have a tool designed to simply return the user_id it receives from its context.

Expected behavior
It should return the user_id of the user calling the agent,

but, in current situation we are getting tmp_user

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2409 from akshaypachpute-1998:fix-issue-2407-agent-tool-context-propogation 0c3e8656fdf11386e3ab13a3a1f2df99a396dbd1
PiperOrigin-RevId: 798315832

src/google/adk/tools/agent_tool.py

@@ -135,7 +135,7 @@ class AgentTool(BaseTool):
     )
     session = await runner.session_service.create_session(
         app_name=self.agent.name,
-        user_id='tmp_user',
+        user_id=tool_context._invocation_context.user_id,
         state=tool_context.state.to_dict(),
     )