fix(tests): hoist vi.mock to top level in vitest setup

[Himarghya]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Description

In vitest.setup.ts, the vi.mock('astro:content', ...) call was nested inside a beforeAll hook. Vitest hoists mock calls automatically, which triggered the following deprecation warning during test execution:

Warning: A vi.mock("astro:content") call in "vitest.setup.ts" is not at the top level of the module. Although it appears nested, it will be hoisted and executed before any tests run. Move it to the top level to reflect its actual execution order.

This PR moves the mock call to module scope (top-level) and removes the unused beforeAll import to follow Vitest best practices and eliminate the warning.

Verification

• Tested locally with pnpm test (all 129 tests passed across 7 test files with zero warnings)
• Checked formatting and linting with pnpm format and pnpm lint
• Verified static build with pnpm build