test(models): exercise real connect() path in test_connect

[anxkhn]

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:

test_connect patched the method it is meant to exercise. It did
mock.patch("google.adk.models.google_llm.Gemini.connect", ...) at the class
level, so the real connect() body never ran and the only assertion was
connection is mock_connection, i.e. it checked that the test's own mock
returned the mock the test itself created. That is a tautology: it passes no
matter how connect() actually behaves, so the live-connect path
(live_connect_config assembly, the _live_api_client.aio.live.connect(...)
call, and GeminiLlmConnection construction) has effectively no coverage from
the test named after it.

Solution:

Patch the _live_api_client boundary instead and let the real connect() body
run, matching how the neighboring connect tests already work
(test_connect_with_custom_headers, test_connect_without_custom_headers,
test_connect_forwards_thinking_config). The test now asserts the live client
is called once with the request model and live_connect_config, and that
connect() yields a GeminiLlmConnection wrapping the live session with the
expected _api_backend and _model_version. This is a test-only change; no
production code is modified.

Testing Plan

Unit Tests:

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

$ pytest tests/unittests/models/test_google_llm.py::test_connect
1 passed

$ pytest tests/unittests/models/test_google_llm.py -k connect
8 passed, 58 deselected

$ pytest tests/unittests/models/test_google_llm.py
66 passed

To confirm the rewritten test is no longer tautological, I temporarily
introduced three regressions in connect() and each was caught: yielding the
raw session instead of a GeminiLlmConnection, passing a fresh
LiveConnectConfig() instead of the request's, and setting a wrong
model_version. The old version of the test passed all three.

Manual End-to-End (E2E) Tests:

Not applicable. This is a unit-test-only change to an existing test; it exercises
Gemini.connect() with the live API client mocked at its boundary and adds no
new user-facing behaviour.

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. (N/A - test-only change.)
• Any dependent changes have been merged and published in downstream modules. (N/A.)