feat: add remote branch diff and merge client APIs (#3686)

This PR adds some support for `diff` / `merge` in the remote client as
for local tables we stay `NotSupported` until
https://github.com/lance-format/lance/issues/7263.


This wires the two review-and-land calls against the remote REST API:
- `POST /v1/table/{id}/branches/diff`
- `POST /v1/table/{id}/branches/merge`

Rust gets typed results (`BranchDiff`, `MergeBranchResult`). Python
returns the wire JSON, same shape as the REST response.

Merge here means promoting a branch's added columns onto `main`.

### Behavior
- Remote only. Local raises `NotSupported`.
- A rejected merge is not an exception. HTTP 409 still returns `Ok` / a
dict with `status="rejected"` and blockers in `diff.mergeBlockers`.
- Unknown blocker / status codes parse as `Unknown` so a newer server
does not break older clients.
- `MergePreview` tolerates missing fields for the same reason.
- Merge requests are not retried. 409 is final and carries the body you
need.

### Example
```python

table = db.open_table("images")

table.branches.create("exp")
exp = table.branches.checkout("exp")

exp.add_columns({"tag": "cast('draft' as string)"})

diff = table.branches.diff("exp")
preview = table.branches.merge("exp", dry_run=True)
result = table.branches.merge("exp", dry_run=False)

if result["status"] == "merged":
    print("landed at", result["mainVersionAfter"])
elif result["status"] == "rejected":
    print(result["diff"]["mergeBlockers"])
```

### Testing
cargo test -p lancedb --features remote diff_branch
cargo test -p lancedb --features remote merge_branch

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

nodejs/src/table.rs

@@ -1355,4 +1355,28 @@ impl Branches {
     pub async fn delete(&self, name: String) -> napi::Result<()> {
         self.inner.delete_branch(&name).await.default_error()
     }
+
+    #[napi(ts_return_type = "Promise<Record<string, unknown>>")]
+    pub async fn diff(&self, from_branch: String) -> napi::Result<serde_json::Value> {
+        let diff = self.inner.diff_branch(&from_branch).await.default_error()?;
+        serde_json::to_value(diff).map_err(|err| {
+            napi::Error::from_reason(format!("failed to serialize branch diff: {err}"))
+        })
+    }
+
+    #[napi(ts_return_type = "Promise<Record<string, unknown>>")]
+    pub async fn merge(
+        &self,
+        from_branch: String,
+        dry_run: Option<bool>,
+    ) -> napi::Result<serde_json::Value> {
+        let result = self
+            .inner
+            .merge_branch(&from_branch, dry_run.unwrap_or(false))
+            .await
+            .default_error()?;
+        serde_json::to_value(result).map_err(|err| {
+            napi::Error::from_reason(format!("failed to serialize branch merge result: {err}"))
+        })
+    }
 }