FIX: run only the Psychosocial sub-harms whose datasets were picked

[Utkarsh Bahuguna (u7k4rs6)]

Fixes #3085.

_selected_sub_harms() now keeps only the requested sub-harms whose dataset is in the dataset config. Validation, seed resolution, the estimate and the attack build all go through it, so a dataset subset narrows all of them. If none of the requested sub-harms has its dataset, it raises a DatasetConstraintError that names the missing dataset.

The default config has both datasets, so default runs and sub_harm alone behave the same as before. Compound configs still hit the existing "compound children" error.

Same repro on this branch:

requested sources: ['airt_licensed_therapist']
run datasets: ['airt_licensed_therapist']
atomic attacks: ['licensed_therapist']

Tests: a therapist-only config builds and estimates only that sub-harm, and sub_harm=imminent_crisis with only the therapist dataset raises. Both fail on main. tests/unit/scenario and backend: 3815 passed.