Upgrade Vitest to 4.x across monorepo

[nnoce14]

Objective:

• Upgrade all workspace packages in the Cellix monorepo (pnpm workspaces) to use Vitest 4.x for testing.

Scope:

• Update all relevant dependencies and peer dependencies to align with Vitest 4.x requirements.
• Refactor or fix any test or config file incompatibilities introduced by Vitest 4.x (see package and workspace tsconfigs, test configs in each package).
• Verify all tests pass in all packages after upgrade (pnpm run test, pnpm run test:coverage).
• Update any documentation or setup instructions referencing Vitest usage/commands if changed.

Acceptance Criteria:

• All packages build and tests pass using Vitest 4.x.
• No regressions or test suite failures in CI pipeline (pnpm run verify passes).
• Monorepo doc references (README, etc.) reflect updated Vitest guidance if workflow or config changes.