### Bug
`value_to_sql({...})` builds a DataFusion `named_struct(...)` literal
but interpolates the struct field names directly as `f"'{k}'"`. A field
name that contains a single quote therefore produces invalid SQL:
```python
>>> from lancedb.util import value_to_sql
>>> value_to_sql({"it's": 1})
"named_struct('it's', 1)" # invalid SQL — the quote terminates the literal
```
String *values* are already escaped (single quotes doubled) by the `str`
branch of `value_to_sql`, so keys and values were handled
inconsistently. This affects `Table.update(values={...})` /
`merge_insert` when a struct column has a field name containing `'`.
### Fix
Render the key through `value_to_sql(str(k))` so field names are escaped
exactly like string values:
```python
>>> value_to_sql({"it's": 1})
"named_struct('it''s', 1)"
```
Keys without special characters are unchanged (`'a'` stays `'a'`), so
existing behavior is preserved.
### Verification
```
$ pytest python/tests/test_util.py -k value_to_sql_dict
```
The new `test_value_to_sql_dict_key_escaping` covers quoted keys (incl.
nested structs) and fails on `main` (`named_struct('it's', 1)`), passes
with this change; the existing `test_value_to_sql_dict` still passes.
Co-authored-by: JSap0914 <JSap0914@users.noreply.github.com>
