chore: updating CONTRIBUTING.md with missing extra

Merge google#1679

Contributing doc says to do the following:

```sh
uv sync --extra test --extra eval
pytest ./tests/unittests
```

If you follow this the tests will fail:

```sh
tests/unittests/a2a/executor/test_task_result_aggregator.py:27: in <module>
    from a2a.types import Message
E   ModuleNotFoundError: No module named 'a2a'
```

This makes sense since the `a2a` package is not part of ADK's core dep, it is an extra:

https://github.com/google/adk-python/blob/e79651cd86ba3f0c998109f2140f1db2cab78708/pyproject.toml#L79-L83

Thus for a2a tests to pass we must include the extra in the sync command:

```sh
uv sync --extra test --extra eval --extra a2a
pytest ./tests/unittests
```

COPYBARA_INTEGRATE_REVIEW=google#1679 from jackwotherspoon:main c1a5367
PiperOrigin-RevId: 776617515

Author: jackwotherspoon

CONTRIBUTING.md

@@ -147,11 +147,11 @@ For any changes that impact user-facing documentation (guides, API reference, tu
     pytest ./tests/unittests
     ```
 
-    NOTE: for accurately repro test failure, only include `test` and `eval` as
-    extra dependencies.
+    NOTE: for accurate repro of test failure, only include `test`, `eval` and 
+    `a2a` as extra dependencies.
 
     ```shell
-    uv sync --extra test --extra eval
+    uv sync --extra test --extra eval --extra a2a
     pytest ./tests/unittests
     ```