fix: include app name in FileArtifactService storage paths

[Nant361]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: FileArtifactService stores artifacts outside the app namespace #5618

Problem:

FileArtifactService accepted app_name in its public artifact APIs, but stored file artifacts under root/users/{user_id}/.... This meant the file backend did not isolate artifacts by app name.

Solution:

Add a validated app_name segment to the file storage layout: root/apps/{app_name}/users/{user_id}/....

This change passes app_name through the file path helpers, keeps session-scoped and user-scoped artifact behavior intact, and updates metadata path expectations.

Testing Plan

Unit Tests:

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

Passed locally:

• uv run --extra test pytest tests/unittests/artifacts/test_artifact_service.py -q
• Result: 71 passed in 0.92s

Added coverage:

• Cross-app isolation for session-scoped artifacts.
• Cross-app isolation for user-scoped artifacts.
• Rejection of traversal and invalid values in app_name.

Manual End-to-End (E2E) Tests:

N/A. This change is covered by file artifact service unit 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. N/A for this unit-level file backend change.
• Any dependent changes have been merged and published in downstream modules. N/A.

Additional context

This keeps the file artifact backend aligned with the artifact service API boundary and the other artifact backends.

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