chore(FR-2933): disable vitest cache in CI

[nowgnuesLee]

Resolves #7510 (FR-2933)

Adds --no-cache to the three pnpm exec vitest run --coverage invocations in .github/workflows/vitest.yml (jobs react-vitest, backend-ai-ui-vitest, root-vitest).

Vitest's default on-disk cache (node_modules/.vitest/) is unnecessary in CI — each run starts from a fresh checkout, so the cache cannot meaningfully speed up runs, and a partial cache restore could mask test-selection or skip changes. Disabling it on CI only makes runs explicitly reproducible while keeping local-dev caching unchanged (no edits to any vitest.config.ts).

Verification

• bash scripts/verify.sh — no new failures from this change (the YAML edit is not on any TypeScript, Relay, or Vite path). Pre-existing failures on main (TS ERR_UNKNOWN_BUILTIN_MODULE, Vite warmup paths) are unrelated.
• Workflow YAML is exercised by GitHub Actions on PR open.

[nowgnuesLee]

• chore(FR-2933): disable vitest cache in CI #7512 👈 (View in Graphite)
• main

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[Copilot]

Pull request overview

This PR improves CI reproducibility by disabling Vitest’s on-disk cache during GitHub Actions runs, ensuring test selection and execution are not influenced by any cached state.

Changes:

• Add --no-cache to the React Vitest coverage run in CI.
• Add --no-cache to the packages/backend.ai-ui Vitest coverage run in CI.
• Add --no-cache to the root Vitest coverage run in CI.