chore: rename smoke test script and add testpaths to pytest config

Rename scripts/test_a2a_live.py to scripts/a2a_smoke_test.py to
prevent pytest from collecting it as a test file. Add testpaths =
["tests"] to pyproject.toml to restrict test discovery.

pyproject.toml

@@ -334,3 +334,4 @@ dev = [
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
+testpaths = ["tests"]