Restore atheris stubs (#15140)

stubs/atheris/METADATA.toml

@@ -0,0 +1,10 @@
+version = "2.3.*"
+upstream_repository = "https://github.com/google/atheris"
+partial_stub = true
+
+[tool.stubtest]
+ignore_missing_stub = true
+# TODO (2025-03-05): unskip once `atheris` can be installed on `ubuntu-24.04`,
+# see https://github.com/python/typeshed/pull/13582 and
+# https://github.com/google/atheris/issues/82
+skip = true