fix(artifacts): treat session_id=None as the user namespace - #7599

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:
BaseArtifactService documents session_id=None as the user-scoped namespace ("If None, the artifact is user-scoped"), and so does the artifact service guide ("an optional session_id, where None means the user-scoped namespace"). FileArtifactService follows that: an artifact saved with session_id=None is the same one sessions reach as user:<name>, with shared versions.

InMemoryArtifactService and GcsArtifactService instead raise InputValidationError: Session ID must be provided for session-scoped artifacts. for any filename without the user: prefix, on save, load, list_versions, delete and the version lookups. So the same call works against local files and fails in memory and on GCS:

await service.save_artifact(app_name="app", user_id="u", session_id=None,
                            filename="u.txt", artifact=part)
# FileArtifactService: 0
# InMemoryArtifactService / GcsArtifactService: InputValidationError

Solution:
When session_id is None, both services now route a bare filename to the user namespace, the same storage path user:<name> already uses, instead of raising. Calls that used to fail now behave like FileArtifactService; nothing that worked before changes. app_name/user_id validation is unchanged, and session_id validation still runs whenever a session id is given.

FileArtifactService lists a session-less save under its bare name rather than with the user: prefix. The interface docstring allows that, so I left it alone.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added test_no_session_id_addresses_user_namespace, parametrized over the in-memory, GCS and file services. It saves with session_id=None, reads and re-saves through user: from a session, checks the shared versions and deletes through session_id=None.

# without the fix
pytest tests/unittests/artifacts/test_artifact_service.py::test_no_session_id_addresses_user_namespace
2 failed, 1 passed     (IN_MEMORY and GCS: InputValidationError)

# with the fix
3 passed

pytest tests/unittests/artifacts -n 4
1125 passed

Manual End-to-End (E2E) Tests:

Not run against a real GCS bucket; the GCS case runs on the test suite's mocked client, like the existing GCS tests.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.