fix(python): support nullable pandas merge input (#3864)

## Summary

- add an end-to-end Python regression for pandas DataFrame inputs merged
into a table created from a Pydantic model
- verify reordered, nullable Arrow source fields can update and insert
into a non-nullable target schema when the values contain no nulls

## Root cause

Lance merge_insert previously compared source schema nullability with
the target, unlike add. The upstream fix now pinned by LanceDB ignores
declared nullability during schema compatibility and validates actual
null values at write time. LanceDB lacked regression coverage for the
full pandas-to-Pydantic path, so this test locks in the correct behavior
without falsifying the input schema nullability.

## Validation

- 5 focused merge-insert tests passed
- Ruff lint passed for the repository
- Ruff format check passed for the changed file
- git diff --check passed

Fixes #2366

<!-- lance-gatekeeper-fix:v1 agent=f897fccfa206620c8a2acdc3bcd1c21f
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_table.py

@@ -2387,6 +2387,55 @@ def test_merge_insert(mem_db: DBConnection):
         )
 
 
+def test_merge_insert_nullable_pandas_into_pydantic_schema(mem_db: DBConnection):
+    # Regression test for https://github.com/lancedb/lancedb/issues/2366
+    pd = pytest.importorskip("pandas")
+
+    class Document(LanceModel):
+        id: int
+        title: str
+        content: str
+
+    table = mem_db.create_table("documents", schema=Document)
+    table.add(
+        pd.DataFrame(
+            {
+                "title": ["Old title", "Unchanged"],
+                "id": [2, 3],
+                "content": ["Old content", "Keep this"],
+            }
+        )
+    )
+
+    # Pandas produces nullable Arrow fields, in an order that differs from the
+    # non-nullable Pydantic schema. This is valid as long as the data has no nulls.
+    new_data = pd.DataFrame(
+        {
+            "title": ["Inserted", "Updated"],
+            "id": [1, 2],
+            "content": ["New row", "New content"],
+        }
+    )
+    result = (
+        table.merge_insert("id")
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(new_data)
+    )
+
+    assert result.num_inserted_rows == 1
+    assert result.num_updated_rows == 1
+    expected = pa.Table.from_pylist(
+        [
+            {"id": 1, "title": "Inserted", "content": "New row"},
+            {"id": 2, "title": "Updated", "content": "New content"},
+            {"id": 3, "title": "Unchanged", "content": "Keep this"},
+        ],
+        schema=Document.to_arrow_schema(),
+    )
+    assert table.to_arrow().sort_by("id") == expected
+
+
 def test_merge_insert_by_source_delete_expr(mem_db: DBConnection):
     table = mem_db.create_table(
         "my_table",