feat: support setting unenforced primary key (#3394)

## Summary

Adds `Table::set_unenforced_primary_key` — records a single column as
the
table's unenforced primary key in Lance schema field metadata.
"Unenforced"
means LanceDB does not check uniqueness on write; the key is metadata
that
`merge_insert` consumes.

- Single-column only; the column must exist and have a supported dtype
(Int32, Int64, Utf8, LargeUtf8, Binary, LargeBinary, FixedSizeBinary).
The
API accepts an iterable for binding ergonomics but requires exactly one
  column — compound keys are rejected.
- The primary key is immutable: calling this on a table that already has
an
unenforced primary key is rejected. Concurrent writers racing to set the
key
  fail at commit time rather than silently overriding it.
- `RemoteTable` returns `NotSupported`.
- Bindings: Python (`AsyncTable`, `LanceTable`, `RemoteTable`) and
TypeScript
  (`Table.setUnenforcedPrimaryKey`).

## Context

Split out from #3354 per review feedback, so the unenforced primary key
and the
`merge_insert` sharding spec land as separate reviewable PRs.

No Lance dependency bump — `main` is already on v7.0.0-beta.10, which
includes
the field-metadata round-trip fix the API relies on. Enforcing
primary-key
immutability at the Lance commit layer (so the cross-column concurrent
race is
also rejected) is a companion Lance change: lance-format/lance#6810.

python/src/table.rs

@@ -805,6 +805,19 @@ impl Table {
         })
     }
 
+    pub fn set_unenforced_primary_key<'a>(
+        self_: PyRef<'a, Self>,
+        columns: Vec<String>,
+    ) -> PyResult<Bound<'a, PyAny>> {
+        let inner = self_.inner_ref()?.clone();
+        future_into_py(self_.py(), async move {
+            inner
+                .set_unenforced_primary_key(columns)
+                .await
+                .infer_error()
+        })
+    }
+
     pub fn uses_v2_manifest_paths(self_: PyRef<'_, Self>) -> PyResult<Bound<'_, PyAny>> {
         let inner = self_.inner_ref()?.clone();
         future_into_py(self_.py(), async move {