fix(versioning): skip non-clean parents in force-parent-dirty hook

The force-parent-dirty listener was calling attributes.flag_modified
on every parent reachable from a dirty child — including parents
themselves in session.new (e.g. brand-new SqlaTable + brand-new
TableColumns from POST /api/v1/dataset/). flag_modified rejects
unloaded attributes, and a session.new SqlaTable's uuid (default=uuid4
fires at flush time) is unloaded until then. CI caught this with
InvalidRequestError cascading into 422s across dataset creation /
upload / Playwright dataset specs.

The hook is only needed for the persistent-and-clean case (child
edited, parent's own scalars untouched, dropdown otherwise empty).
Anything in session.new will flush anyway; anything in session.dirty
is already flagged; session.deleted shouldn't be touched. Short-
circuit before the flag_modified call.

Unblocks test-sqlite, test-mysql, test-postgres (previous), and
playwright dataset specs.