⬆️ 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.

client/src/hooks/Conversations/usePresetIndexOptions.ts

@@ -41,7 +41,7 @@ const usePresetIndexOptions: TUsePresetOptions = (_preset) => {
   const setExample: TSetExample = (i, type, newValue = null) => {
     const update = {};
     const current = preset?.examples?.slice() || [];
-    const currentExample = { ...current[i] } || {};
+    const currentExample = { ...current[i] };
     currentExample[type] = { content: newValue };
     current[i] = currentExample;
     update['examples'] = current;