test: increase lightclient e2e test timeout to prevent flaky failures (ChainSafe#8841)

## Description

Fixes flaky lightclient e2e test failures introduced in ChainSafe#8825.

### Root Cause

The `waitForBestUpdate()` function can take up to **7000ms**:
- 5 slots waiting for `lightClientOptimisticUpdate` event (5000ms)  
- 2 slots sleep (2000ms)

But the default vitest timeout is **5000ms**, causing race condition
failures on slower CI machines.

### Evidence

Local test run showed `getLightClientUpdatesByRange()` taking
**7384ms**:
```
✓ getLightClientUpdatesByRange() 7384ms
✓ getLightClientOptimisticUpdate() 5982ms  
✓ getLightClientCommitteeRoot() for the 1st period 6003ms
```

### Fix

Increases timeout to 10s for tests using `waitForBestUpdate()`.

---

> [!NOTE]
> This PR was authored by Lodekeeper (AI assistant) under supervision of
@nflaig.

---------

Co-authored-by: lodekeeper <lodekeeper@users.noreply.github.com>
Co-authored-by: Nico Flaig <nflaig@protonmail.com>

Author: lodekeeper

packages/beacon-node/test/e2e/api/impl/lightclient/endpoint.test.ts

@@ -1,4 +1,4 @@
-import {afterEach, beforeEach, describe, expect, it} from "vitest";
+import {afterEach, beforeEach, describe, expect, it, vi} from "vitest";
 import {HttpHeader, getClient, routes} from "@lodestar/api";
 import {ChainConfig, createBeaconConfig} from "@lodestar/config";
 import {ForkName} from "@lodestar/params";
@@ -13,6 +13,8 @@ import {getDevBeaconNode} from "../../../../utils/node/beacon.js";
 import {getAndInitDevValidators} from "../../../../utils/node/validator.js";
 
 describe("lightclient api", () => {
+  vi.setConfig({testTimeout: 10_000});
+
   const SLOT_DURATION_MS = 1000;
   const restPort = 9596;
   const ELECTRA_FORK_EPOCH = 0;