test: stabilize Insights sample bundle loading

[Raafet57]

Summary

• Fixes the post-merge main CI failure from run 25372579744.
• Stabilizes InsightsPage.test.tsx sample-bundle interactions by waiting for the manifest-loaded select to be enabled and for the requested option to exist before user.selectOptions.
• Test-only change; no product/runtime behavior changed.

Failure addressed

pnpm test failed on main after PR #8 merge because the sample-bundle test tried to select linked while the async manifest was still in the Loading bundles… placeholder state.

Verification

• pnpm test -- src/pages/InsightsPage.test.tsx repeated 3x — passed, 27 files / 200 tests each run.
• pnpm test — passed, 27 files / 200 tests.
• pnpm format
• pnpm verify — passed: lint, format check, typecheck, unit tests, privacy audit, deterministic data checks, build.
• pnpm test:e2e — passed, 18/18.
• npx -y @google/design.md lint DESIGN.md — 0 errors; existing unused-token warnings only.
• git diff --check — passed.
• Codex implementation review: APPROVED.

Boundaries

• No runtime/product code changed.
• No backend, upload path, telemetry, persistence, deployment, release, tag, or publication.