fix: avoid false-positive testing-library/prefer-presence-queries

The eslint-plugin-testing-library prefer-presence-queries rule
auto-fixes `service.getById(...)` to `service.queryById(...)` when
inside `expect(...).toBeNull()`, mistaking our service method for a
Testing Library query. Extracting the call into a variable breaks the
pattern match without changing semantics or the public API.

src/platform/downloads/providers/createBrowserDownloadService.test.ts

@@ -48,7 +48,8 @@ describe('createBrowserDownloadService', () => {
 
   it('getById returns null', () => {
     const service = createBrowserDownloadService()
-    expect(service.getById('anything')).toBeNull()
+    const entry = service.getById('anything')
+    expect(entry).toBeNull()
   })
 
   it('onProgress returns a no-op unsubscribe', () => {