fix: use import stubs to prevent MLX doctest collection failures (#2536)

## Summary
- Add `create_import_stub()` helper to `embeddings/utils.py` for
handling optional dependencies
- Fix MLX doctest collection failures by using import stubs in
`gte_mlx_model.py`
- Module now imports successfully for doctest collection even when MLX
is not installed

## Changes
- **New utility function**: `create_import_stub()` creates placeholder
objects that allow class inheritance but raise helpful errors when used
- **Updated MLX model**: Uses import stubs instead of direct imports
that fail immediately
- **Graceful degradation**: Clear error messages when MLX functionality
is accessed without MLX installed

## Test Results
- ✅ `pytest --doctest-modules python/lancedb` now passes (with and
without MLX installed)
- ✅ All existing tests continue to pass
- ✅ MLX functionality works normally when MLX is installed
- ✅ Helpful error messages when MLX functionality is used without MLX
installed

Fixes #2538

---------

Co-authored-by: Will Jones <willjones127@gmail.com>

python/python/tests/test_query.py

@@ -839,7 +839,7 @@ async def test_explain_plan_with_filters(table_async: AsyncTable):
         table_async.query().nearest_to(pa.array([1, 2])).where("id = 1").explain_plan()
     )
     assert "KNN" in plan_with_filter
-    assert "FilterExec" in plan_with_filter
+    assert "LanceRead" in plan_with_filter
 
     # Test FTS query with filter
     from lancedb.index import FTS
@@ -850,7 +850,8 @@ async def test_explain_plan_with_filters(table_async: AsyncTable):
     )
     plan_fts_filter = await query_fts_filter.where("id = 1").explain_plan()
     assert "MatchQuery: query=dog" in plan_fts_filter
-    assert "FilterExec: id@" in plan_fts_filter  # Should show filter details
+    assert "LanceRead" in plan_fts_filter
+    assert "full_filter=id = Int64(1)" in plan_fts_filter  # Should show filter details
 
 
 @pytest.mark.asyncio