fix: when_not_matched_by_source_delete() doesn't reset a previously-set condition (#3771)

## Summary

`LanceMergeInsertBuilder.when_not_matched_by_source_delete()` didn't
clear a previously-set condition when called again with no argument (or
a different condition type). Per the docstring, `condition=None` means
"delete all unmatched rows," but if the builder had already been
configured with a string/Expr condition, a later no-arg call left the
stale condition in place instead of widening the delete to
unconditional.

Fixes #3767

## Change

Each call now unconditionally sets both
`_when_not_matched_by_source_condition` and
`_when_not_matched_by_source_condition_expr` (one to the new value, the
other to `None`), so the latest call always wins — consistent with every
other setter on this builder (e.g.
`when_matched_update_all(where=...)`).

## Test plan

- [x] New regression test
`test_merge_insert_by_source_delete_reconfigure` in
`python/python/tests/test_table.py`
- [x] `uv run --extra tests pytest
python/tests/test_table.py::test_merge_insert_by_source_delete_reconfigure
python/tests/test_table.py::test_merge_insert_by_source_delete_expr
python/tests/test_table.py::test_merge_insert_by_source_delete_expr_async
-vv` — 3 passed
- [x] `uv run --extra dev ruff format` / `ruff check` — clean

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

python/python/lancedb/merge.py

@@ -92,8 +92,10 @@ class LanceMergeInsertBuilder(object):
         self._when_not_matched_by_source_delete = True
         if isinstance(condition, Expr):
             self._when_not_matched_by_source_condition_expr = condition._inner
-        elif condition is not None:
+            self._when_not_matched_by_source_condition = None
+        else:
             self._when_not_matched_by_source_condition = condition
+            self._when_not_matched_by_source_condition_expr = None
         return self
 
     def use_index(self, use_index: bool) -> LanceMergeInsertBuilder:

python/python/tests/test_table.py

@@ -2364,6 +2364,29 @@ def test_merge_insert_by_source_delete_expr(mem_db: DBConnection):
     assert table.to_arrow().sort_by("a") == expected
 
 
+def test_merge_insert_by_source_delete_reconfigure(mem_db: DBConnection):
+    # Calling when_not_matched_by_source_delete() again with no condition must
+    # widen the delete to unconditional, not keep the earlier condition around.
+    table = mem_db.create_table(
+        "my_table",
+        data=pa.table({"a": [1, 2, 3], "b": ["a", "b", "c"]}),
+    )
+    new_data = pa.table({"a": [2, 4], "b": ["x", "z"]})
+
+    merge_insert_res = (
+        table.merge_insert("a")
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .when_not_matched_by_source_delete("a > 2")
+        .when_not_matched_by_source_delete()
+        .execute(new_data)
+    )
+    assert merge_insert_res.num_deleted_rows == 2
+
+    expected = pa.table({"a": [2, 4], "b": ["x", "z"]})
+    assert table.to_arrow().sort_by("a") == expected
+
+
 @pytest.mark.asyncio
 async def test_merge_insert_by_source_delete_expr_async(
     mem_db_async: AsyncConnection,