fix(importer): use ORM relationship assignment for dashboard_slices

The v1 import pipeline previously wrote dashboard ↔ chart membership
via raw Core DML (``db.session.execute(delete(dashboard_slices)…)`` +
``db.session.execute(insert(dashboard_slices)…)``). With Continuum's
M2M tracker enabled by the versioning feature, those Core writes
emit malformed shadow INSERTs into ``dashboard_slices_version`` —
the tracker can't see the composite-PK columns through the Core
layer and produces rows with only ``(transaction_id, operation_type)``
populated, triggering a ``NOT NULL`` violation on
``(dashboard_id, slice_id)``.

Rewrites both import paths (``ImportAssetsCommand._import`` in
``commands/importers/v1/assets.py`` and ``ImportDashboardsCommand._import``
in ``commands/dashboard/importers/v1/__init__.py``) to use ORM-level
``dashboard.slices = [...]`` reassignment followed by an explicit
``db.session.flush()``. The explicit flush is necessary to land the
M2M rows before any subsequent autoflush fires an inner-flush event
handler that would reset the relationship change (cf. the SAWarning
``Attribute history events accumulated on N previously clean instances
within inner-flush event handlers have been reset``).

The unit tests previously called ``_import`` directly twice in the same
session — production wraps ``run()`` in ``@transaction`` so each invocation
gets its own DB+Continuum transaction. Added ``db.session.commit()`` between
calls in ``test_import_adds_dashboard_charts``,
``test_import_removes_dashboard_charts``, and
``test_dashboard_import_with_overwrite_replaces_charts`` so the tests
mirror production semantics; otherwise the second call's M2M shadow
inserts conflict with the first call's on
``UNIQUE (dashboard_id, slice_id, transaction_id)``.