Validation warning with BackendLookupFailure in crossbackend mode

[soxofaan]

When submitting a job with collections on separate upstream backends, but with "crossbackend" feature enabled, the user still sees a validation warning about "Collections across multiple backends" because validation does not take the "crossbackend" job option into account.

bbox = {"west": 3, "south": 51, "east": 3.01, "north": 51.01}
cube1 = connection.load_collection(
    "ESA_WORLDCEREAL_WINTERCEREALS",
    spatial_extent=bbox,
    temporal_extent="2024-08",
)
cube2 = connection.load_collection(
    "SENTINEL2_L2A",
    spatial_extent=bbox,
    temporal_extent="2024-08",
)
cube = cube1.merge_cubes(cube2)

job = cube.create_job(
    job_options={"split_strategy": "crossbackend"}
)

the auto-validation feature of the python client triggers this warning from create_job:

Preflight process graph validation raised: [InternalValidationFailure] Validation failed: BackendLookupFailureException(status_code=400, code='BackendLookupFailure', message="Collections across multiple backends ({'cdse', 'terrascope'}): {'SENTINEL2_L2A', 'ESA_WORLDCEREAL_WINTERCEREALS'}.", id='r-240903d69117456082baea6867c5f3c3')

The validation warning is not blocking at the moment, but unnecessary and confusing to the user