Fix PR CI Check

Goal

Get PR #4882 CI green by fixing the actual failing job, not guessing.

Phases

• Inspect PR head and failing job
• Pull failing logs and relevant prior learnings
• Reproduce the failure locally
• Implement the fix
• Verify with the relevant gate

Findings

• PR head matches local HEAD, so CI is not behind a missing push.
• Prior repo learnings already cover barrel drift, helper-driven module mocks, and build fallback races.
• The current CI failure is pnpm test:slowest, not lint, build, or typecheck.
• Three fast-lane React specs crossed the hard 75ms/test line in CI and need the slow lane.
• Local prediction needed a warning band below CI thresholds; otherwise fast machines hide drift.
• tooling/scripts/test-slow.mjs had the same helper-driven mock.module(...) pollution risk that test-fast.mjs already solved.
• One moved callout slow spec also needed local spies instead of shared module replacement.

Verification

• pnpm prepare
• pnpm test:slow -- packages/ai/src/react/ai-chat/hooks/useChatChunk.slow.tsx packages/callout/src/react/hooks/useCalloutEmojiPicker.slow.tsx packages/selection/src/react/hooks/useSelectionArea.slow.tsx
• pnpm turbo build --filter=./packages/callout
• pnpm turbo typecheck --filter=./packages/callout
• pnpm lint:fix
• pnpm install
• bun check