feat(remote): support set_unenforced_primary_key (#4158)

## Summary

`RemoteTable::set_unenforced_primary_key` returned `NotSupported`, so
the call failed against LanceDB Cloud and enterprise from every SDK —
Python and TypeScript both forward to it. That also blocked sharded LSM
writes, since the server rejects bucket/identity sharding on a table
that declares no unenforced primary key, and the SDK had no way to
declare one.

No server-side support was missing. The unenforced primary key is Lance
schema field metadata, and the existing `update_field_metadata` endpoint
writes exactly that, so the remote table now installs the key through
it. The commit layer behind that endpoint installs the position,
enforces immutability and runs `verify_primary_key()` — the same code a
native table reaches, so both paths agree on semantics and not just on
error messages.

## Changes

- `rust/lancedb/src/table/primary_key.rs`: the request validation
(single column, column exists, supported dtype, key not already set) and
the metadata edit (set the 1-indexed position, clear the legacy boolean
key) move into `validate()` and `install_edit()`, shared by both table
types so they cannot drift. Native behaviour is unchanged.
- `rust/lancedb/src/remote/table.rs`: real implementation —
`check_mutable()`, validate against the described schema, then one
`update_field_metadata` request.
- `python/python/lancedb/remote/table.py`: drop the "Not supported on
LanceDB Cloud." docstring.

No binding changes were needed; Python and TypeScript already forward to
the Rust `BaseTable` method.

## Tests

Three mocked-server tests in `rust/lancedb/src/remote/table.rs`:

- the happy path, asserting the request body carries
`lance-schema:unenforced-primary-key:position: "1"` and a null legacy
key on the right field path;
- the invalid requests a native table also rejects (empty, compound,
unknown column, unsupported dtype), each failing before any write
reaches the server;
- the already-set case, rejected from the schema the server already
reports.

`cargo test --features remote -p lancedb --lib`: 1186 passed, 0 failed.
`cargo clippy --features remote -p lancedb --lib --tests` is clean.

## Note for enterprise

Sophon pins the lancedb submodule at `v0.39.0-beta.6`, so this reaches
enterprise on the next bump.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_011xV8EebEZDmDDV8hVBm6MD

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

python/python/lancedb/remote/table.py

@@ -1034,7 +1034,7 @@ class RemoteTable(Table):
         return LOOP.run(self._table.drop_columns(columns))
 
     def set_unenforced_primary_key(self, columns: Union[str, Iterable[str]]) -> None:
-        """Not supported for remote tables."""
+        """Set the unenforced primary key for this table to a single column."""
         return LOOP.run(self._table.set_unenforced_primary_key(columns))
 
     def set_lsm_write_spec(self, spec: "LsmWriteSpec") -> None:

python/python/tests/test_remote_db.py

@@ -1623,7 +1623,6 @@ def remote_unsupported_operations():
     return [
         ("uses_v2_manifest_paths", (), {}, "uses_v2_manifest_paths"),
         ("migrate_v2_manifest_paths", (), {}, "migrate_manifest_paths_v2"),
-        ("set_unenforced_primary_key", (["id"],), {}, "set_unenforced_primary_key"),
         ("fetch_blob_ranges", ("image", [(1, 0, 1)]), {}, "fetch_blob_ranges"),
         ("optimize", (), {}, "optimize"),
         ("create_index", ("vector",), {"config": HnswPq()}, "IVF_HNSW_PQ"),