feat(python): support dict to SQL struct conversion in table.update() (#3089)

## Summary

- Add `@value_to_sql.register(dict)` handler that converts Python dicts
to DataFusion's `named_struct()` SQL syntax
- Enables updating struct-typed columns via `table.update(values={"col":
{"field_a": 1, "field_b": "hello"}})`
- Recursively handles nested structs, lists, nulls, and all existing
scalar types

Closes #1363

## Details

The `named_struct` function was introduced in DataFusion 38 and is now
available (LanceDB uses DataFusion 52.1). The implementation follows the
existing `singledispatch` pattern in `util.py`.

**Example conversion:**
```python
value_to_sql({"field_a": 1, "field_b": "hello"})
# => "named_struct('field_a', 1, 'field_b', 'hello')"
```

## Test plan

- [x] Unit tests for flat struct, nested struct, list inside struct,
mixed types, null values, and empty dict
- [ ] CI integration tests with actual table.update() on struct columns

🔗 [DataFusion named_struct
docs](https://datafusion.apache.org/user-guide/sql/scalar_functions.html#named-struct)

python/python/tests/test_util.py

@@ -121,6 +121,32 @@ def test_value_to_sql_string(tmp_path):
         assert table.to_pandas().query("search == @value")["replace"].item() == value
 
 
+def test_value_to_sql_dict():
+    # Simple flat struct
+    assert value_to_sql({"a": 1, "b": "hello"}) == "named_struct('a', 1, 'b', 'hello')"
+
+    # Nested struct
+    assert (
+        value_to_sql({"outer": {"inner": 1}})
+        == "named_struct('outer', named_struct('inner', 1))"
+    )
+
+    # List inside struct
+    assert value_to_sql({"a": [1, 2]}) == "named_struct('a', [1, 2])"
+
+    # Mixed types
+    assert (
+        value_to_sql({"name": "test", "count": 42, "rate": 3.14, "active": True})
+        == "named_struct('name', 'test', 'count', 42, 'rate', 3.14, 'active', TRUE)"
+    )
+
+    # Null value inside struct
+    assert value_to_sql({"a": None}) == "named_struct('a', NULL)"
+
+    # Empty dict
+    assert value_to_sql({}) == "named_struct()"
+
+
 def test_append_vector_columns():
     registry = EmbeddingFunctionRegistry.get_instance()
     registry.register("test")(MockTextEmbeddingFunction)