sessions endpoint returns sessions where datasets don't exist on Flatiron

[k1o0]

We want to use the sessions endpoint to find sessions where a given list of dataset types exist on Flatiron. Currently this is not the case.

From @oliche:

I think the issue comes from the fact that the dataset_type filter on the session endpoint does not enforce the exist flag, unlike the dataset serializer in the session endpoint.
This consistency issue needs fixing in the custom filter (actions.views I think)

[k1o0]

This appears to still be an issue. Sessions with no datasets where exist=True are returned when using the datasets filter. For example GET /sessions?datasets=_iblrig_taskSettings.raw.json&date_range=2025-01-01%2C2025-01-14