feat(remote): implement set/unset_lsm_write_spec REST variant (#3501)

## Summary

Wires `RemoteTable::set_lsm_write_spec` / `unset_lsm_write_spec` to the
sophon REST endpoints added in
[lancedb/sophon#6181](https://github.com/lancedb/sophon/pull/6181),
replacing the previous `NotSupported` stubs.

- `set_lsm_write_spec` maps the `LsmWriteSpec` onto sophon's request DTO
— mode-tagged `sharding` (`unsharded` / `bucket` / `identity`),
`maintained_indexes`, and `writer_config_defaults` — and POSTs to
`/v1/table/{name}/set_lsm_write_spec/`.
- `unset_lsm_write_spec` POSTs to
`/v1/table/{name}/unset_lsm_write_spec/`.
- Both call `check_mutable` first, matching the other remote mutations.
- `maintained_indexes` is sent verbatim (an empty list means "no
maintained indexes", matching native semantics).

## Testing

- Added mocked-endpoint unit tests for unsharded / bucket / identity set
and for unset.
- `cargo check --features remote --tests` passes.

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

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

rust/lancedb/src/remote/table.rs

@@ -1827,16 +1827,57 @@ impl<S: HttpSend> BaseTable for RemoteTable<S> {
         })
     }
 
-    async fn set_lsm_write_spec(&self, _spec: crate::table::LsmWriteSpec) -> Result<()> {
-        Err(Error::NotSupported {
-            message: "set_lsm_write_spec is not supported on LanceDB cloud.".into(),
-        })
+    async fn set_lsm_write_spec(&self, spec: crate::table::LsmWriteSpec) -> Result<()> {
+        use crate::table::LsmWriteSpec;
+        self.check_mutable().await?;
+
+        // Map the spec onto the server's request DTO. `sharding` is internally
+        // tagged on `mode` to mirror sophon's `Sharding` enum; `maintained_indexes`
+        // and `writer_config_defaults` are sent verbatim (an empty list means "no
+        // maintained indexes", not "default to all").
+        let sharding = match &spec {
+            LsmWriteSpec::Bucket {
+                column,
+                num_buckets,
+                ..
+            } => serde_json::json!({
+                "mode": "bucket",
+                "column": column,
+                "num_buckets": num_buckets,
+            }),
+            LsmWriteSpec::Identity { column, .. } => serde_json::json!({
+                "mode": "identity",
+                "column": column,
+            }),
+            LsmWriteSpec::Unsharded { .. } => serde_json::json!({ "mode": "unsharded" }),
+        };
+        let body = serde_json::json!({
+            "sharding": sharding,
+            "maintained_indexes": spec.maintained_indexes(),
+            "writer_config_defaults": spec.writer_config_defaults(),
+        });
+
+        let request = self
+            .client
+            .post(&format!(
+                "/v1/table/{}/set_lsm_write_spec/",
+                self.identifier
+            ))
+            .json(&body);
+        let (request_id, response) = self.send(request, true).await?;
+        self.check_table_response(&request_id, response).await?;
+        Ok(())
     }
 
     async fn unset_lsm_write_spec(&self) -> Result<()> {
-        Err(Error::NotSupported {
-            message: "unset_lsm_write_spec is not supported on LanceDB cloud.".into(),
-        })
+        self.check_mutable().await?;
+        let request = self.client.post(&format!(
+            "/v1/table/{}/unset_lsm_write_spec/",
+            self.identifier
+        ));
+        let (request_id, response) = self.send(request, true).await?;
+        self.check_table_response(&request_id, response).await?;
+        Ok(())
     }
 
     async fn tags(&self) -> Result<Box<dyn Tags + '_>> {
@@ -4428,6 +4469,91 @@ mod tests {
         assert!(matches!(e, Error::IndexNotFound { .. }));
     }
 
+    #[tokio::test]
+    async fn test_set_lsm_write_spec_unsharded() {
+        let table = Table::new_with_handler("my_table", |request| {
+            assert_eq!(request.method(), "POST");
+            assert_eq!(
+                request.url().path(),
+                "/v1/table/my_table/set_lsm_write_spec/"
+            );
+            let body = request.body().unwrap().as_bytes().unwrap();
+            let body: serde_json::Value = serde_json::from_slice(body).unwrap();
+            assert_eq!(body["sharding"], serde_json::json!({ "mode": "unsharded" }));
+            assert_eq!(body["maintained_indexes"], serde_json::json!(["id_idx"]));
+            assert_eq!(
+                body["writer_config_defaults"],
+                serde_json::json!({ "max_memtable_rows": "1000" })
+            );
+            http::Response::builder()
+                .status(200)
+                .body(r#"{"maintained_indexes":["id_idx"]}"#)
+                .unwrap()
+        });
+        let spec = crate::table::LsmWriteSpec::unsharded()
+            .with_maintained_indexes(["id_idx"])
+            .with_writer_config_defaults([("max_memtable_rows", "1000")]);
+        table.set_lsm_write_spec(spec).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn test_set_lsm_write_spec_bucket() {
+        let table = Table::new_with_handler("my_table", |request| {
+            assert_eq!(request.method(), "POST");
+            assert_eq!(
+                request.url().path(),
+                "/v1/table/my_table/set_lsm_write_spec/"
+            );
+            let body = request.body().unwrap().as_bytes().unwrap();
+            let body: serde_json::Value = serde_json::from_slice(body).unwrap();
+            assert_eq!(
+                body["sharding"],
+                serde_json::json!({ "mode": "bucket", "column": "id", "num_buckets": 16 })
+            );
+            assert_eq!(body["maintained_indexes"], serde_json::json!([]));
+            http::Response::builder().status(200).body("{}").unwrap()
+        });
+        table
+            .set_lsm_write_spec(crate::table::LsmWriteSpec::bucket("id", 16))
+            .await
+            .unwrap();
+    }
+
+    #[tokio::test]
+    async fn test_set_lsm_write_spec_identity() {
+        let table = Table::new_with_handler("my_table", |request| {
+            assert_eq!(request.method(), "POST");
+            assert_eq!(
+                request.url().path(),
+                "/v1/table/my_table/set_lsm_write_spec/"
+            );
+            let body = request.body().unwrap().as_bytes().unwrap();
+            let body: serde_json::Value = serde_json::from_slice(body).unwrap();
+            assert_eq!(
+                body["sharding"],
+                serde_json::json!({ "mode": "identity", "column": "tenant" })
+            );
+            http::Response::builder().status(200).body("{}").unwrap()
+        });
+        table
+            .set_lsm_write_spec(crate::table::LsmWriteSpec::identity("tenant"))
+            .await
+            .unwrap();
+    }
+
+    #[tokio::test]
+    async fn test_unset_lsm_write_spec() {
+        let table = Table::new_with_handler("my_table", |request| {
+            assert_eq!(request.method(), "POST");
+            assert_eq!(
+                request.url().path(),
+                "/v1/table/my_table/unset_lsm_write_spec/"
+            );
+            http::Response::builder().status(200).body("{}").unwrap()
+        });
+        table.unset_lsm_write_spec().await.unwrap();
+    }
+
     #[tokio::test]
     async fn test_wait_for_index() {
         let table = _make_table_with_indices(0);