TUI coverage gate flakes on Windows: App.test.tsx OAuth step-up tests fail under v8 instrumentation only

[olaservo]

Version: v2 (main, reproduced at 07a2b4b)
Affected client: clients/tui (test infrastructure; Windows only)

Symptom

On Windows 11, npm run test:coverage in clients/tui fails with exactly one test failing per run — but a different one each time, always in App.test.tsx > App (OAuth result branches):

• run 1: shows insufficient-scope message when step-up OAuth stays insufficient
• run 2: completes a standard step-up authorize when OAuth succeeds
• run 3 (clean upstream tree): same as run 2

npx vitest run __tests__/App.test.tsx without coverage passes 80/80 every time. The failures only appear under v8 instrumentation, which slows the Ink render loop enough for these waitFor-style OAuth-branch assertions to race.

What's known

• Pre-exists unrelated work: reproduced identically on the upstream tip with no local changes.
• Linux CI is green, so it's Windows-manifested (slower instrumented renders under the runner here).
• 3/3 instrumented runs failed; 0 failures uninstrumented.

Impact

coverage:tui → root coverage → npm run ci fails on Windows even on a clean tree. Together with #1939 (verify scripts can't spawn npx) and #1941 (CLI worker teardown crash), the mandatory pre-push gate currently cannot pass on a Windows machine.

Context

Found while validating the fix for #1852 on a Windows machine.