Use PolarRequestValidationError for seats/units subscription update errors

[stilla]

Subscription seat/unit update validation errors (NotASeatBasedSubscription, BelowMinimumSeats, AboveMaximumSeats, SeatsAlreadyAssigned, and the unit-based equivalents introduced in #13871) are plain SubscriptionError/PolarError subclasses returning a flat 400 response, unlike the rest of server/polar/subscription/service.py which uses PolarRequestValidationError for a structured 422 response.

Convert both the seats and units update validation paths (validate_seats_change, validate_units_change in server/polar/subscription/service.py) to raise PolarRequestValidationError instead, for consistency.

Raised from this discussion.

_{Sent by @frankie567 from PR #13871 dedicated issue creation.}