Add mount invariant test for FleetJobPaths COS-to-container paths

[avilches]

Summary

Adds test_cos_to_container_mount_invariant to test_fleet_utils.py to verify that every (cos_key, cos_prefix) pair in FleetJobPaths resolves to the expected container path after PDS mounting.

The invariant: if a COS key is {prefix}/{filename} and prefix is mounted at mount_point, the container path must be mount_point/{filename}. Currently nothing catches a change to one side (e.g. renaming a COS prefix) without updating the other (the container path), so this test closes that gap.

The test covers all 9 relevant pairs across both custom jobs (4 cases) and provider jobs (5 cases, including the private log) in a single table-driven assertion:

for label, cos_key, prefix, mount, container in cases:
    relative = cos_key.removeprefix(prefix + "/")
    assert container == f"{mount}/{relative}", f"mount invariant failed for '{label}'"

[marceloamaral]

LGTM

The base branch was changed.

[avilches]

Tests were green, which shows that the code was correct. Now I'm changing the PR to add the refactor while keeping the test unchanged. I'm resolving conflicts and squash commit because the branch base change.

[avilches]

Refactor looks horrible, so close it. So, I would just keep this test only.

[avilches]

@ElePT @marceloamaral quick approve to merge plis?