fix(python): reject embedding source updates without vectors

Fixes #4761

`Table.update` does not compute embeddings, so changing an embedding source
column could leave the previous embedding in place. Source-column updates now
raise `ValueError` unless every associated vector column is supplied in the same
call.

The guard reads Python embedding metadata and native `lancedb::column_definitions`
without loading embedding models. It uses Lance's field-path parser for quoted
targets and covers synchronous and asynchronous local and remote tables. Supplying
every associated vector column, vector-only updates, and unrelated updates remain
supported.

This is a breaking change for source-only updates. The API documentation and
Update rows guide explain the requirement and the `merge_insert` alternative for
regenerating embeddings.

Validation at the reviewed head covered 44 targeted public update cases and the
full Python suite (3107 passed, 162 skipped, 70 deselected). All applicable
exact-head checks and workflows completed successfully.

python/src/lib.rs

@@ -128,6 +128,7 @@ pub fn _lancedb(py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(table::tokenize, m)?)?;
     m.add_function(wrap_pyfunction!(permutation::async_permutation_builder, m)?)?;
     m.add_function(wrap_pyfunction!(util::validate_table_name, m)?)?;
+    m.add_function(wrap_pyfunction!(util::parse_field_path, m)?)?;
     m.add_function(wrap_pyfunction!(query::fts_query_to_json, m)?)?;
     m.add_function(wrap_pyfunction!(expr_col, m)?)?;
     m.add_function(wrap_pyfunction!(expr_lit, m)?)?;

python/src/util.rs

@@ -61,6 +61,12 @@ pub fn validate_table_name(table_name: &str) -> PyResult<()> {
         .map_err(|e| PyValueError::new_err(e.to_string()))
 }
 
+/// Parse field references with the same identifier syntax as Lance writes.
+#[pyfunction(name = "_parse_field_path")]
+pub fn parse_field_path(path: &str) -> PyResult<Vec<String>> {
+    lance_core::datatypes::parse_field_path(path).map_err(|e| PyValueError::new_err(e.to_string()))
+}
+
 /// A wrapper around a LanceDB type to allow it to be used in Python
 #[derive(Debug, Clone)]
 pub struct PyLanceDB<T>(pub T);