fix(a2a): stop to_a2a from overriding the application's log level

[ankit2235]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: to_a2a() overrides the google_adk logger level set by the application #7462

Problem:
to_a2a() sets the google_adk logger to INFO every time it's called. Any level the application configured beforehand gets overwritten, e.g. a uvicorn --log-config that sets google_adk to DEBUG loses all ADK debug output once the module calls to_a2a(), and a WARNING level starts letting INFO through again.

The comment said the call was there so logs show up when running under uvicorn directly, but uvicorn's default config doesn't attach a root handler, so ADK INFO records weren't printed with or without it.

Solution:
Remove the setLevel call and leave log levels to the application. The adk_logger variable stays since two warnings further down still use it.

One behavior change to be aware of: an app that sets up its own root handler but never sets a level on google_adk will no longer see ADK's INFO lines after calling to_a2a(). That matches what the app configured, but if you'd prefer to keep INFO as a default, I can change this to only set the level when google_adk is still NOTSET.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added test_to_a2a_keeps_configured_adk_log_level. It fails on main (assert 20 == 30, i.e. INFO instead of WARNING) and passes with the fix.

pytest tests/unittests/a2a/utils/test_agent_to_a2a.py -q
46 passed, 2 skipped

pytest tests/unittests/a2a -q
892 passed, 51 skipped

Manual End-to-End (E2E) Tests:

A small module that logs at DEBUG before and after calling to_a2a(), served with uvicorn and a log config that sets google_adk to DEBUG:

log = logging.getLogger("google_adk.probe")
log.debug("DEBUG before to_a2a")
a2a_app = to_a2a(LlmAgent(name="probe", model="gemini-2.5-flash"), port=8799)
log.debug("DEBUG after to_a2a")

uvicorn app_mod:a2a_app --port 8799 --log-config log.json

Before the fix:

DEBUG before to_a2a

After the fix:

DEBUG before to_a2a
DEBUG after to_a2a

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.