feat: support checking out a version on a branch (#3504)

### Description

Stacked on #3490. Adds an optional version to branch checkout across the
Rust core and the Python and TypeScript SDKs, so you can open a specific
version on a branch ("version V of branch B"), not just the branch's
latest version

Rust

```rust
// Open version 3 of branch "exp" (a read-only view): check out from an
// existing table, or open it directly from the connection.
let exp_v3 = table.checkout_branch("exp", Some(3)).await?;
let exp_v3 = db.open_table("items").branch("exp").version(3).execute().await?;
// checkout_latest re-attaches to the branch's writable HEAD.
exp_v3.checkout_latest().await?;

// With no branch, a version opens main at that version.
let main_v3 = db.open_table("items").version(3).execute().await?;
```

Python

```python
# Open version 3 of branch "exp" (a read-only view): check out from an
# existing table, or open it directly from the connection.
branch_v3 = await table.branches.checkout("exp", version=3)
branch_v3 = await db.open_table("items", branch="exp", version=3)
# checkout_latest re-attaches to the branch's writable HEAD.
await branch_v3.checkout_latest()

# With no branch, a version opens main at that version.
main_v3 = await db.open_table("items", version=3)
```

TypeScript

```typescript
// Open version 3 of branch "exp" (a read-only view): check out from an
// existing table, or open it directly from the connection.
const branchV3 = await (await table.branches()).checkout("exp", 3);
const opened = await db.openTable("items", undefined, { branch: "exp", version: 3 });
// checkoutLatest re-attaches to the branch's writable HEAD.
await branchV3.checkoutLatest();

// With no branch, a version opens main at that version.
const mainV3 = await db.openTable("items", undefined, { version: 3 });
```

### Testing
- Added unit tests (Rust, Python sync + async, TypeScript):
branch-scoped resolution at a version number shared with `main` and with
another branch, read-only enforcement on a pinned handle,
`checkout_latest` recovery to the branch's HEAD, fork-point reads, and
the nonexistent-version/branch error paths.
- Ran smoke tests against the Python and TypeScript SDKs on local
machine.

nodejs/__test__/remote.test.ts

@@ -191,6 +191,34 @@ describe("remote connection", () => {
     );
   });
 
+  it("allows version on remote but rejects a non-main branch", async () => {
+    await withMockDatabase(
+      (_req, res) => {
+        // describe (table open + version validation) always succeeds
+        const body = JSON.stringify({
+          name: "t",
+          version: 2,
+          schema: { fields: [] },
+        });
+        res.writeHead(200, { "Content-Type": "application/json" }).end(body);
+      },
+      async (db) => {
+        // version-only (and "main" + version) is allowed: remote supports
+        // version time-travel even though it has no branches
+        await db.openTable("t", undefined, { version: 2 });
+        await db.openTable("t", undefined, { branch: "main", version: 2 });
+
+        // a non-main branch is rejected, with or without a version
+        await expect(
+          db.openTable("t", undefined, { branch: "exp" }),
+        ).rejects.toThrow(/branching/);
+        await expect(
+          db.openTable("t", undefined, { branch: "exp", version: 2 }),
+        ).rejects.toThrow(/branching/);
+      },
+    );
+  });
+
   describe("TlsConfig", () => {
     it("should create TlsConfig with all fields", () => {
       const tlsConfig: TlsConfig = {