⬆️ chore: Bump TypeScript to 5.9.3 (+ typescript-eslint 8.60.1) (#13584)

Bumps typescript 5.3.3 -> 5.9.3 across all workspaces. typescript-eslint must move 8.24.0 -> 8.60.1 too: 8.24's typescript peer was capped at <5.8.0; 8.60.1 widens it to <6.1.0.

Two errors surfaced by the newer compiler are fixed:
- api/src/rum/proxy.ts: TS 5.9 made `Buffer` generic (`Buffer<ArrayBufferLike>`), which no longer structurally matches `BodyInit`; cast the fetch body (Node's fetch accepts a Buffer at runtime).
- client usePresetIndexOptions.ts: drop a dead `|| {}` on an object spread (always truthy — flagged by the new TS2872 check).

All four package typecheck jobs + the client app typecheck pass under 5.9.3; builds (tsdown + rollup) and the rum proxy tests are unaffected.

packages/api/src/rum/proxy.ts

@@ -1,5 +1,5 @@
-import type { Request, Response } from 'express';
 import { logger } from '@librechat/data-schemas';
+import type { Request, Response } from 'express';
 import { isEnabled } from '~/utils';
 
 const DEFAULT_PROXY_PATH = '/api/rum';
@@ -127,7 +127,9 @@ export async function proxyRumRequest(req: Request, res: Response): Promise<void
     const response = await fetch(target, {
       method: 'POST',
       headers: getProxyHeaders(req, body),
-      body,
+      // TS 5.9 made `Buffer` generic (`Buffer<ArrayBufferLike>`), which no longer
+      // structurally matches `BodyInit`; Node's fetch accepts a Buffer body at runtime.
+      body: body as BodyInit,
       signal: controller.signal,
     });