feat: add user_id in readonly context

[nag763]

Hey

I want to access a Firestore database where documents are retrieved given my user ID.

So far, it is a bit tedious to access the user ID calling a tool, while it could be simplified by adding a RO attribute on the ReadOnly parent context of most Callback/Tools.

If there are easier ways to access the caller's user ID, kindly let me know as a reply of this.

Thanks a lot!

EDIT: I did type this in a rush, rephrased

[nag763]

pytest report

$ uv run pytest  tests/unittests/ 
=============================================== test session starts ===============================================
platform linux -- Python 3.11.12, pytest-8.3.5, pluggy-1.6.0
rootdir: /home/usr/workspace/adk-python
configfile: pyproject.toml
plugins: langsmith-0.3.43, asyncio-1.0.0, xdist-3.7.0, mock-3.14.1, anyio-4.9.0
asyncio: mode=Mode.AUTO, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 1621 items        


...

================================= 1616 passed, 5 skipped, 170 warnings in 15.01s ==================================

Before on #8d36dbda520b1c0dec148e1e1d84e36ddcb9cb95 1614 passed, 5 skipped, 170 warnings in 14.66s

[nag763]

Actually, the user id is not preserved while an agent uses another one as a toll, as it passes tmp_user instead of parent[session.user_id]

adk-python/src/google/adk/tools/agent_tool.py

Line 134 in da4bc0e

user_id='tmp_user',

Anyone can provide guidance on why it was implemented that way and what alternatives do exist at the moment to get user ID from a running tool ?