test(common.storage): cover async-only registry + factory contract

The drop-sync refactor is now contractually enforced:

* create_storage({"mode": "sync"|"async"|<any>}) raises StorageConfigError
  -- the regression we most want to catch is someone "restoring" the
  sync shape by re-introducing mode= handling.
* create_storage() preserves caller's dict, returns an AsyncStorageBackend
  subclass, and wraps constructor TypeError into a useful StorageConfigError.
* build_storage_config emits no mode key on either local or s3 branch.
* register_backend() refuses to silently overwrite a different class on
  the same name (collision guard), and accepts same-class re-registration.
* registry surfaces the built-in local + s3 classes after import; the
  conflict-test cleanup pattern avoids leaking global state across tests.

Adds [tool.pytest.ini_options] (asyncio_mode=auto, testpaths=tests) so
uv run --package common pytest common/tests works from the workspace root.

17 new tests, all green. Backend suite still 136 passed.

common/pyproject.toml

@@ -31,3 +31,7 @@ default = true
 dev = [
     "pytest>=9.1.1",
 ]
+
+[tool.pytest.ini_options]
+asyncio_mode = "auto"
+testpaths = ["tests"]