Store each conversation chain in a separate FoundryStateStore.
Store creation and reuse follow FoundryStateStore.get_or_create semantics.
user_isolation, item_ttl_seconds, description, and tags apply
only when the underlying store is first created. Existing stores retain
their settings; neither mismatch policy updates them.
By default, each read and write fetches the store properties and fails before
item access if isolation or TTL differs from this instance's options. Set
on_mismatch="ignore" to skip this validation and use existing settings.
In that mode, passing user_isolation=True does not enable isolation on
an existing non-isolated store; callers must ensure its settings are suitable.