fix: require a primary key to enable MemWAL (#4561)

## Why

Every MemWAL write and read resolves rows by the table's unenforced
primary key, but `enable_memwal` accepted a table without one. The spec
installed, and every MemWAL `merge_insert` then failed with "table has a
MemWAL index but no unenforced primary key" until MemWAL was disabled.
Lance's MemWAL initialization doesn't catch it: its key checks pass when
there is no key.

## What changes

`enable_memwal` refuses a spec on a table with no unenforced primary
key, for every sharding mode, before anything is committed. The check
runs against the dataset the spec is committed to; a key cannot be
removed once set, and an overwrite or restore of the table conflicts
with that commit. Servers that install specs through this method get the
check too.

The Rust doc on `Table::enable_memwal` now states the requirement; the
Python and TypeScript docs already did.

## Tests

- New `test_enable_memwal_requires_primary_key` (Rust and Python):
unsharded, bucket and identity specs are all refused without a key, a
refused spec leaves nothing installed, and the same table accepts a spec
once a key is set. The Rust test fails with the check disabled.
- Tests that installed a spec on a table without a key now declare one
first (6 Rust, 3 Python).
- `cargo nextest run -p lancedb --features remote --lib`: 1529 passed.
`cargo clippy -p lancedb --features remote --tests --examples -- -D
warnings` and `cargo fmt --check` are clean; no integration test or
example installs a spec.
- Python: `test_memwal_spec.py` and `test_merge_insert_lsm.py`, 43
passed; `ruff format` and `ruff check` are clean.

python/python/tests/test_memwal_spec.py

@@ -45,6 +45,7 @@ def _make_table(tmp_path):
 
 def test_enable_memwal_validates(tmp_path):
     _db, table = _make_table(tmp_path)
+    table.set_unenforced_primary_key("id")
 
     # Out-of-range num_buckets.
     with pytest.raises(Exception, match="num_buckets"):
@@ -60,6 +61,7 @@ def test_enable_memwal_validates(tmp_path):
 
 def test_disable_memwal(tmp_path):
     _db, table = _make_table(tmp_path)
+    table.set_unenforced_primary_key("id")
 
     # unset errors when no spec is set.
     with pytest.raises(Exception, match="no MemWAL write spec"):
@@ -74,6 +76,18 @@ def test_disable_memwal(tmp_path):
     table.enable_memwal(MemWalWriteSpec.bucket("id", 8))
 
 
+def test_enable_memwal_requires_primary_key(tmp_path):
+    _db, table = _make_table(tmp_path)
+    for spec in (
+        MemWalWriteSpec.unsharded(),
+        MemWalWriteSpec.bucket("id", 4),
+        MemWalWriteSpec.identity("id"),
+    ):
+        with pytest.raises(Exception, match="primary key"):
+            table.enable_memwal(spec)
+    assert table.get_memwal_spec() is None
+
+
 def test_set_unsharded_spec(tmp_path):
     _db, table = _make_table(tmp_path)
     # Lance MemWAL still requires a primary key on the dataset; Unsharded
@@ -206,6 +220,7 @@ async def test_async_get_memwal_spec(tmp_path):
     )
 
     assert await table.get_memwal_spec() is None
+    await table.set_unenforced_primary_key("id")
 
     # A real scalar index is needed to name it as a maintained index.
     await table.create_index("id", config=BTree())