OnDiskStorage.delete(period) deletes only that exact period, not the periods within it #564

Holder.delete_arrays(period) is documented to remove "all values for any period included in period (e.g. if period is "2017", values for "2017-01", "2017-07", etc. would be removed)". InMemoryStorage.delete does that (period.contains(...)). OnDiskStorage.delete instead removes only the file whose key is exactly f"{branch_name}_{period}" (policyengine_core/data_storage/on_disk_storage.py, delete). So the same deletion removes monthly values from memory but leaves them on disk. A disk-backed simulation then keeps reading values the caller deleted.

Reproduction on 3.32.12 (master b78b0ba), country template only:

"""OnDiskStorage.delete(period) deletes only that exact period (policyengine-core 3.32.12)."""
import warnings
warnings.simplefilter("ignore")
from policyengine_core.country_template import CountryTaxBenefitSystem
from policyengine_core.experimental import MemoryConfig
from policyengine_core.simulations import SimulationBuilder


def simulation(on_disk):
    sim = SimulationBuilder().build_from_entities(
        CountryTaxBenefitSystem(),
        {"persons": {"a": {}}, "households": {"h": {"parents": ["a"]}}},
    )
    if on_disk:
        sim.memory_config = MemoryConfig(max_memory_occupation=0)
        holder = sim.get_holder("rent")
        holder._disk_storage = holder.create_disk_storage()
        holder._on_disk_storable = True
    sim.set_input("rent", "2025-01", [500.0])
    sim.set_input("rent", "2025-02", [600.0])
    sim.delete_arrays("rent", "2025")
    return [str(p) for p in sim.get_holder("rent").get_known_periods()]


print("in memory, after delete_arrays('rent', '2025'):", simulation(False))
print("on disk,   after delete_arrays('rent', '2025'):", simulation(True))

Output:

in memory, after delete_arrays('rent', '2025'): []
on disk,   after delete_arrays('rent', '2025'): ['2025-01', '2025-02']

Suggested fix: in OnDiskStorage.delete, when period is given, parse each key with the same split #552 introduces (key.rsplit("_", 1), since a period's string form never contains _), and delete the files whose branch is branch_name and whose period period.contains, as InMemoryStorage.delete does. Eternal storage keeps deleting its single ETERNITY key. Best done after #552 merges, since #552 adds that key parser and fixes the period-None branch's prefix match. Test exact, containing, excluding and eternal periods on both backends.

Found while fixing #559 (PR #561). #561 keeps _user_input_keys in step with whatever disk storage actually keeps, so it does not depend on this fix. The PolicyEngine-UK #2025 round-3 review (finding 5) hit the same behaviour.