feat(client): Table.load_columns() REST client for LOAD COLUMNS

Geneva Table.load_columns() parity on the REST-only client. Fills existing
columns from an external Parquet/Lance/IPC source by primary-key join.

- BaseTable::load_columns default (NotSupported) + public Table::load_columns,
  taking a LoadColumnsRequest (source uris/format/storage_options, target/source
  key, (target, source?) column mappings, on_missing, worker/batch/commit knobs).
- Remote impl POSTs to /v1/table/{id}/load_columns with the matching body;
  mock test asserts the request shape.
- PyO3 binding + Python remote Table.load_columns(source, pk, columns, *,
  source_format, source_pk, on_missing, ...) accepting a column list or
  {target: source} dict.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

python/src/table.rs

@@ -17,8 +17,8 @@ use arrow::{
     pyarrow::{FromPyArrow, PyArrowType, ToPyArrow},
 };
 use lancedb::table::{
-    AddDataMode, ColumnAlteration, Duration, FieldMetadataUpdate, NewColumnTransform,
-    OptimizeAction, OptimizeOptions, Ref, Table as LanceDbTable,
+    AddDataMode, ColumnAlteration, Duration, FieldMetadataUpdate, LoadColumnsRequest,
+    NewColumnTransform, OptimizeAction, OptimizeOptions, Ref, Table as LanceDbTable,
 };
 use pyo3::{
     Bound, FromPyObject, Py, PyAny, PyRef, PyResult, Python,
@@ -1100,6 +1100,43 @@ impl Table {
         })
     }
 
+    #[allow(clippy::too_many_arguments)]
+    #[pyo3(signature = (source_uris, source_format, target_key, columns, source_key=None, source_storage_options=None, on_missing=None, num_workers=None, max_workers=None, batch_size=None, commit_granularity=None, priority=None))]
+    pub fn load_columns(
+        self_: PyRef<'_, Self>,
+        source_uris: Vec<String>,
+        source_format: String,
+        target_key: String,
+        columns: Vec<(String, Option<String>)>,
+        source_key: Option<String>,
+        source_storage_options: Option<std::collections::HashMap<String, String>>,
+        on_missing: Option<String>,
+        num_workers: Option<u32>,
+        max_workers: Option<u32>,
+        batch_size: Option<u32>,
+        commit_granularity: Option<u32>,
+        priority: Option<String>,
+    ) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner_ref()?.clone();
+        let request = LoadColumnsRequest {
+            source_uris,
+            source_format,
+            source_storage_options,
+            target_key,
+            source_key,
+            columns,
+            on_missing,
+            num_workers,
+            max_workers,
+            batch_size,
+            commit_granularity,
+            priority,
+        };
+        future_into_py(self_.py(), async move {
+            inner.load_columns(request).await.infer_error()
+        })
+    }
+
     pub fn add_columns(
         self_: PyRef<'_, Self>,
         definitions: Vec<(String, String)>,