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>

docs/src/js/classes/Branches.md

@@ -83,6 +83,24 @@ Delete a branch.
 
 ***
 
+### diff()
+
+```ts
+diff(fromBranch): Promise<BranchDiff>
+```
+
+Compare a branch against main without modifying either branch.
+
+#### Parameters
+
+* **fromBranch**: `string`
+
+#### Returns
+
+`Promise`&lt;[`BranchDiff`](../interfaces/BranchDiff.md)&gt;
+
+***
+
 ### list()
 
 ```ts
@@ -94,3 +112,28 @@ List all branches, mapping name to branch metadata.
 #### Returns
 
 `Promise`&lt;`Record`&lt;`string`, [`BranchContents`](BranchContents.md)&gt;&gt;
+
+***
+
+### merge()
+
+```ts
+merge(fromBranch, dryRun): Promise<MergeBranchResult>
+```
+
+Merge a branch into main.
+
+Set `dryRun` to `true` to preview the merge. A rejected merge resolves
+with `status: "rejected"` instead of throwing.
+
+#### Parameters
+
+* **fromBranch**: `string`
+    Branch to merge from.
+
+* **dryRun**: `boolean` = `false`
+    When true, only preview the merge. Defaults to false.
+
+#### Returns
+
+`Promise`&lt;[`MergeBranchResult`](../interfaces/MergeBranchResult.md)&gt;