fix(computer-use-mcp): bound waitForElement frame timeouts (#1856)

## Summary\n- Pass the remaining waitForElement budget into each
frame-level CU_ACTION send so unresponsive frames cannot consume the
fixed 8s sendMessage timeout.\n- Use the remaining deadline for each
poll and stop polling immediately when the budget is exhausted.\n-
Reduce the bridge-side waitForElement grace from the legacy 9.5s buffer
to a small transport grace.\n- Add a regression test covering the
hanging-extension case.\n\n## Validation\n- pnpm -C
services/computer-use-mcp exec vitest run --config ./vitest.config.ts
src/browser-dom/extension-bridge.test.ts

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Co-authored-by: Neko <neko@ayaka.moe>
Co-authored-by: 刘梓恒 <160735726+3361559784@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>