feat: add table branch support to remote tables and Python/TS bindings (#3540)

### Description
Adding branch support for RemoteTable by threading a branch selector
onto every operation the data plane accepts it on. Exposes the
currentBranch to nodejs and python through the bindings.

Matching the server handlers, the branch rides as:
- a `?branch=` query parameter for Arrow-body and query-only ops
(insert, merge_insert, multipart_*, version/list, drop_index)
- a `branch` field in the JSON body for everything else (count_rows,
query, update, delete, create_index, column ops, index list/stats,
stats, restore, describe, tags create/update)

A main-branch handle (`branch == None`) produces byte-identical requests
to before: no `branch` field and no `?branch=`

- Handle-per-branch: `create_branch` / `checkout_branch` return a new
handle with fresh caches and reset version/freshness state, mirroring
`NativeTable`.
- `create_branch` maps 409 to already-exists, 400 to invalid, and 404 to
not-found with source context, and sends without retry so the 409 stays
observable.
- `Ref` translation covers version, version-number (relative to the
handle's branch), and tag (resolved via the tags endpoint); `"main"` and
empty normalize to the main branch.
- Python branch handles persist their branch (and pinned version) across
pickle/fork, so a forked or pickled handle reopens on its branch rather
than silently reverting to main.

### Tests

- Rust mock tests per op category (query-param and body mechanisms,
branch CRUD, error paths, backward-compat).
- Python sync branch CRUD, `open_table(branch=)`, and a pickle
round-trip regression test.

nodejs/src/table.rs

@@ -487,6 +487,12 @@ impl Table {
         })
     }
 
+    /// The branch this handle is scoped to, or `null` for the main branch.
+    #[napi]
+    pub fn current_branch(&self) -> napi::Result<Option<String>> {
+        Ok(self.inner_ref()?.current_branch())
+    }
+
     #[napi(catch_unwind)]
     pub async fn optimize(
         &self,