fix: remove primary key constraint from MemWAL bucket sharding

Lance v7.0.0-rc.1 intentionally removed the requirement for
bucket_sharding to match (or even require) the unenforced primary key
column. Update LanceDB to match: drop the PK-related doc comments and
the test assertions that expected rejection when no PK is set or when
the bucket column differs from the PK.

The Rust changes are taken from #3435; this commit additionally applies
the equivalent updates to the Python and TypeScript bindings.

See https://github.com/lance-format/lance/issues/6917

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

python/src/table.rs

@@ -185,7 +185,7 @@ pub struct LsmWriteSpec {
 
 #[pymethods]
 impl LsmWriteSpec {
-    /// Hash-bucket sharding by the unenforced primary key column.
+    /// Hash-bucket sharding by a scalar column.
     #[staticmethod]
     pub fn bucket(column: String, num_buckets: u32) -> Self {
         Self {