Fix: Snapshot parsing during migration

[izeigerman]

Fix for:

>>> from sqlmesh.core.context import Context
>>>
>>> ctx = Context()
>>> ctx.migrate()
Traceback (most recent call last):
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1218, in migrate
    self._migrate_rows(promoted_snapshots_only)
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1317, in _migrate_rows
    snapshot_mapping = self._migrate_snapshot_rows(snapshots_to_migrate)
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1462, in _migrate_snapshot_rows
    _visit(root_snapshot_id, {})
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1380, in _visit
    snapshot = parsed_snapshots[snapshot_id]
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1762, in __getitem__
    snapshot = self.get(snapshot_id)
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/core/state_sync/engine_adapter.py", line 1753, in get
    self._parsed_snapshots[snapshot_id] = Snapshot.parse_obj(raw_snapshot)
  File "/Users/georgesittas/Code/tobiko/sqlmesh/sqlmesh/utils/pydantic.py", line 181, in parse_obj
    super().model_validate(obj) if PYDANTIC_MAJOR_VERSION >= 2 else super().parse_obj(obj)  # type: ignore
  File "/Users/georgesittas/Code/tobiko/sqlmesh/.venv/lib/python3.10/site-packages/pydantic/main.py", line 503, in model_validate
    return cls.__pydantic_validator__.validate_python(
pydantic_core._pydantic_core.ValidationError: 1 validation error for Snapshot
updated_ts
  Field required [type=missing, input_value={'name': '"db"."sqlmesh_e...913', 'migrated': False}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.5/v/missing
    ```

[izeigerman]

Because of this the existing test didn't catch the issue.