test: settle the ChiefOfStaff page mount before querying it (#5857)

[atomantic]

Summary

client/src/pages/ChiefOfStaff.test.jsx passed in isolation but flaked under a full client run, timing out in findByRole('button', { name: /Force Evaluate/i }). The page fans out several mocked reads on mount and each resolution is its own macrotask, so a bare findBy* straight after render polls blind through that whole chain — on a contended worker it ran past Testing Library's async budget before the config panel had rendered.

Fixed the settle, not the budget:

• Every test that reaches into a tab's contents now mounts through one renderSettledAt(tab) helper that waits for the page's own settle signal — the loading branch's role="status" busy region labelled "Loading Chief of Staff" — to clear. The following query then spends its budget on a single render instead of the entire mount chain. This also collapses four copies of the MemoryRouter/Routes block that had accumulated across describes.
• The two loading-skeleton guards keep mounting bare: they hold a core read open on purpose so the busy branch is what renders.
• asyncUtilTimeout (3s, QuotaBurn retry-budget test flakes ~1 in 3 on testing-library's 1000ms default timeout #3474), the two-worker client cap, and testTimeout are all untouched — raising them was already spent on this file twice and a third bump would slow every genuinely-hung assertion in the suite.

The Force Evaluate button's own contract moved down to ConfigTab.test.jsx, where it renders directly with no page mount to wait on: one onEvaluate call per click, the explanatory title, no API call or toast of its own, and availability while the settings editor is open. The page suite keeps the half that is genuinely page state — the toast and status-bubble result of the handler the button invokes.

Nothing was dropped in the move: assertions across the two files went from 141 to 148.

Test plan

• cd client && npx vitest run — green on three consecutive full runs (831 passed | 1 skipped).
• npx vitest run src/pages/ChiefOfStaff.test.jsx src/components/cos/tabs/ConfigTab.test.jsx — 49 passed (was 47).
• git diff origin/main...HEAD touches only the two test files; client/src/test/setup.js and client/vitest.config.js are unchanged.

Follow-up filed as #6060 for an unrelated bug hit while running the configured local reviewer.

Closes #5857