Restore failing test `test_<service>_api_version_header_async`

[parthea]

FAILED tests/unit/gapic/showcase_v1beta1/test_echo.py::test_echo_api_version_header_async - Failed: async def functions are not natively supported.

Generated test test_<service>_api_version_header_async is failing because the async test code is not marked with @pytest.mark.asyncio

It's not clear why this test started failing, but to avoid blocking development, we will comment out the test until it's fixed

[vchudnov-g]

Note that in draft PR #2426, which I created earlier today, that test failure is solved, but that then revealed (caused?) other tests to fail.

[vchudnov-g]

Note that after the latest commit to #2426, now this async test really is failing (before it was not even running, apparently) for the IAM methods, with a message TypeError: object _StreamingResponseIterator can't be used in 'await' expression. So it seems we may have a real async error either in the implementation or in the the test. See https://github.com/googleapis/gapic-generator-python/actions/runs/17281829015/job/49051607168?pr=2426#step:7:70

@ohmayr for any insights.

[vchudnov-g]

Because of the possible real error, I marked this as a P1 bug.

(If that failing test output is somehow misleading, we can lower to P2 process.)