fix(artifacts): reject the reserved segment 'user' as a session_id

[MUHAMMEDHAFEEZ]

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Link to Issue or Description of Change

• Closes: InMemoryArtifactService/GcsArtifactService collide with the reserved "user" segment when session_id is literally "user" #7063

Problem:

InMemoryArtifactService and GcsArtifactService lay session-scoped and
user-scoped artifacts out in the same flat namespace, using the literal
segment "user" to mark user-scoped artifacts:
_artifact_path/_get_blob_prefix return
f"{app_name}/{user_id}/user/{filename}" for user-namespaced files, and
f"{app_name}/{user_id}/{session_id}/{filename}" otherwise. The reserved
literal "user" is never validated against, so a session actually named
"user" writes its artifacts into the same prefix reserved for
user-scoped ones.

This isn't just a naming collision: list_artifact_keys() for an
unrelated session incorrectly returns filenames that belong to the
"user"-named session, and loading one of those filenames then returns
None — the listing and the load disagree.

Solution:

Adds artifact_util.validate_session_id_segment(), which runs the existing
validate_path_segment() checks and additionally rejects the literal value
"user". Applied at the two places each service already ran
validate_path_segment on session_id:
_artifact_path/_get_blob_prefix and each service's list_artifact_keys.

FileArtifactService is not touched: it lays session-scoped artifacts out
under their own sessions/<id>/ subtree, distinct from the user-scoped
artifacts/ subtree, so a session named "user" cannot collide with it
there (confirmed with a positive test below).

Version Bump?

• Yes
• No

No pyproject.toml version field exists for this package.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing Plan

Unit Tests:

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

Added to tests/unittests/artifacts/test_artifact_service.py:

• test_save_artifact_rejects_reserved_user_as_session_id (IN_MEMORY,
  GCS) — session_id="user" raises InputValidationError.
• test_file_allows_reserved_user_as_session_id — FILE is unaffected and
  a session named "user" saves/loads normally there.
• test_list_artifact_keys_rejects_reserved_user_as_session_id (IN_MEMORY,
  GCS) — the same rejection applies to listing, not just save/load.

Added to tests/unittests/artifacts/test_artifact_util.py:

• test_validate_session_id_segment_rejects_reserved_user
• test_validate_session_id_segment_allows_ordinary_values
• test_validate_session_id_segment_still_runs_path_segment_checks

Confirmed all 5 backend-affecting new tests fail against the unfixed code
(temporarily reverted just the three source files, keeping the new tests)
and pass with the fix restored.

$ pytest tests/unittests/artifacts tests/unittests/sessions
1184 passed, 3 xfailed, 10 warnings

Also ran pyink --check, ruff check, isort --check, codespell, and
mypy on all five changed files. Two pre-existing, unrelated findings
confirmed present on unmodified main (not introduced by this change, left
alone): an unused SimpleNamespace import in test_artifact_service.py,
and a mypy attr-defined error on google.cloud.storage in
gcs_artifact_service.py (missing type stub in this environment).

Manual End-to-End (E2E) Tests:

Ran the reproduction from the issue directly against
InMemoryArtifactService:

Before the fix:

list_artifact_keys(session_id="s2"): ['from_session_user.txt', 'user:shared.txt']
load_artifact(session_id="s2", filename="from_session_user.txt"): None

After the fix: save_artifact(session_id="user", ...) raises
InputValidationError up front, so this state can no longer arise.

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.

Additional context

Surfaced during review of #7030 (whitespace normalization, fixed by
#6958) — a related but distinct root cause (a reserved literal value, not
padding), independent of that fix and not blocked by it.