Enforce visibility filter on dag_runs/latest endpoint

[elijahbenizzy]

Summary

Tightens the scoping of the GET /api/v1/dag_runs/latest/ endpoint in the tracking UI backend to honor the visibility contract that the rest of the get endpoints in this module already follow (documented at the top of trackingserver_base/permissions/permissions.py: "Inside any 'get' endpoints, we only access the items to which the user has visibility").

Previously, when neither project_id nor dag_template_id was supplied, the permission callback short-circuited to "allow" and the endpoint body produced an unfiltered query — returning runs from across the instance instead of the caller's accessible scope. This change makes the no-args case behave the same as every other get endpoint in the module.

Changes

• trackingserver_base/permissions/permissions.py
  • Adds a visible_project_ids_for_user helper that returns the set of project IDs the caller can see (mirrors _get_visible_projects but returns IDs only, so it composes with __in= filters).
  • Updates the user_can_get_latest_dag_runs permission callback so it always evaluates visibility, including the no-args case.
  • Docstring updated to point to the invariant.
• trackingserver_run_tracking/api.py
  • When neither project_id nor dag_template_id is supplied, the endpoint now injects a dag_template__project_id__in=<visible_ids> filter.
  • Behavior when an explicit project_id or dag_template_id is supplied is unchanged.

Tests

• Unit test for the new visible_project_ids_for_user helper (tests/test_db_methods/test_permissions.py).
• Three HTTP regression tests in tests/test_lifecycle/test_run_tracking.py:
  • Cross-user no-arg request does not return runs from projects the caller cannot see.
  • Cross-user explicit project_id request from a non-member returns 404 (matches existing behavior elsewhere).
  • Legitimate own-project request returns 200.
• Verified the no-arg regression test fails when the endpoint scoping is reverted, so the test actually catches the case.

Adjacent

trackingserver_template/api.py:230 (get_dag_template_catalog) accepts project_id: int = None and has the same project_id is None short-circuit shape in its permission callback (user_can_get_dag_templates). Worth a follow-up to apply the same pattern there. Out of scope for this PR.