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/interfaces/BranchDiff.md

@@ -0,0 +1,129 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchDiff
+
+# Interface: BranchDiff
+
+Read-only comparison of a branch against main.
+
+## Properties
+
+### addedColumns
+
+```ts
+addedColumns: BranchColumnSummary[];
+```
+
+***
+
+### addedIndexes
+
+```ts
+addedIndexes: BranchIndexSummary[];
+```
+
+***
+
+### baseMoved
+
+```ts
+baseMoved: boolean;
+```
+
+***
+
+### branchVersion
+
+```ts
+branchVersion: number;
+```
+
+***
+
+### changedColumns
+
+```ts
+changedColumns: BranchColumnChange[];
+```
+
+***
+
+### fromBranch
+
+```ts
+fromBranch: string;
+```
+
+***
+
+### mainVersion
+
+```ts
+mainVersion: number;
+```
+
+***
+
+### mergeBlockers
+
+```ts
+mergeBlockers: MergeBlocker[];
+```
+
+***
+
+### mergeable
+
+```ts
+mergeable: boolean;
+```
+
+***
+
+### parentVersion
+
+```ts
+parentVersion: number;
+```
+
+***
+
+### removedColumns
+
+```ts
+removedColumns: BranchColumnSummary[];
+```
+
+***
+
+### removedIndexes
+
+```ts
+removedIndexes: BranchIndexSummary[];
+```
+
+***
+
+### rowCountBranch
+
+```ts
+rowCountBranch: number;
+```
+
+***
+
+### rowCountMain
+
+```ts
+rowCountMain: number;
+```
+
+***
+
+### rowSummary
+
+```ts
+rowSummary: BranchRowCountSummary;
+```