Return fields annotated with PrivateStateAttr across state schemas.
Annotations are resolved at runtime, so a schema whose PrivateStateAttr
annotation references a TYPE_CHECKING-only name cannot be inspected. That
schema is skipped with a warning rather than failing the whole agent, because
the caller may own several unrelated schemas -- but the warning matters: a
skipped schema keeps none of its private fields, so they will be forwarded to
and merged back from subagents.
