Test-only refactor continuing #2523. Reuses the shared import-state helper in session-related tests, removes duplicated local save/restore logic, and preserves existing test behavior.
Keeps src.request_models real and restores both sys.modules and parent routes.session_routes package attributes after temporary test stubs. Restores one focused part of the Python CI baseline tracked in #2580.
Split 2/4 of the companion bridge (#863 was 1/4). A paired bearer-token caller
runs as the sandboxed 'api' pseudo-user, so its sessions were stranded in a
separate 'api'-owned silo, invisible to the owner's desktop UI.
Add effective_user(): for a bearer token it resolves to the token's real owner
(request.state.api_token_owner); for cookie sessions it is identical to
get_current_user, so the swap is a no-op for browser users. Route session
ownership/attribution in routes/session_routes.py through it.
Tests (tests/test_session_owner_attribution.py):
- cookie/browser users are unchanged
- a bearer token attributes to its owner; with no owner it does NOT escalate
- _verify_session_owner: a bearer token for owner A cannot verify owner B's
session (404); owner verifies their own; missing -> 404; unauth -> 403
