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>

python/src/table.rs

@@ -1593,4 +1593,40 @@ impl Branches {
             Ok(())
         })
     }
+
+    pub fn diff(self_: PyRef<'_, Self>, from_branch: String) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner.clone();
+        future_into_py(self_.py(), async move {
+            let diff = inner.diff_branch(&from_branch).await.infer_error()?;
+            Python::attach(|py| struct_to_wire_py(py, &diff))
+        })
+    }
+
+    #[pyo3(signature = (from_branch, dry_run=false))]
+    pub fn merge(
+        self_: PyRef<'_, Self>,
+        from_branch: String,
+        dry_run: bool,
+    ) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner.clone();
+        future_into_py(self_.py(), async move {
+            let result = inner
+                .merge_branch(&from_branch, dry_run)
+                .await
+                .infer_error()?;
+            Python::attach(|py| struct_to_wire_py(py, &result))
+        })
+    }
+}
+
+/// Decode a serde value as the wire JSON object (camelCase keys).
+fn struct_to_wire_py(py: Python<'_>, value: &impl serde::Serialize) -> PyResult<Py<PyAny>> {
+    let json = py.import("json")?;
+    Ok(json
+        .call_method1(
+            "loads",
+            (serde_json::to_string(value)
+                .map_err(|e| PyRuntimeError::new_err(format!("failed to serialize json: {e}")))?,),
+        )?
+        .unbind())
 }