Stabilize workspace preview coverage timing

The workspace long-file preview test regularly crosses ten seconds under V8 coverage instrumentation even though the behavior passes. Raising its explicit timeout keeps the coverage lane aligned with the runtime it already observes.

Constraint: PR verification runs the full desktop test suite under coverage, which adds substantial highlighting overhead for this fixture.

Rejected: Change the component rendering path | the failure was a coverage-mode test timeout, not a product regression.

Confidence: high

Scope-risk: narrow

Tested: cd desktop && bun run test -- src/components/workspace/WorkspacePanel.test.tsx --run --coverage --coverage.reporter=json-summary --coverage.reporter=lcov --coverage.reportsDirectory=/tmp/cc-haha-workspace-panel-coverage --testTimeout=20000

Tested: bun run check:native

Tested: bun run check:coverage

Tested: git diff --check

desktop/src/components/workspace/WorkspacePanel.test.tsx

@@ -970,7 +970,7 @@ describe('WorkspacePanel', () => {
       expect(view.getByTestId('workspace-code').textContent).toContain('const line2300 = 2300')
     })
     expect(view.getByRole('button', { name: 'Collapse preview' })).toBeTruthy()
-  }, 10_000)
+  }, 20_000)
 
   it('renders image previews from workspace files', async () => {
     await setWorkspaceState((state) => ({