fix: apply configured headers to 404 responses

[SSDWGG]

Summary

• apply configured server.headers / preview.headers to Vite 404 responses
• keep the existing 404 middleware behavior while setting configured headers before ending the response
• add a playground regression for POST requests that fall through to the dev-server 404 handler

Fixes #22543

Validation

• pnpm exec oxfmt packages/vite/src/node/server/middlewares/notFound.ts packages/vite/src/node/server/index.ts packages/vite/src/node/preview.ts playground/fs-serve/__tests__/commonTests.ts
• pnpm exec eslint --cache --concurrency auto packages/vite/src/node/server/middlewares/notFound.ts packages/vite/src/node/server/index.ts packages/vite/src/node/preview.ts playground/fs-serve/__tests__/commonTests.ts
• pnpm --filter vite build
• pnpm exec vitest run -c vitest.config.e2e.ts playground/fs-serve/__tests__/fs-serve.spec.ts

[SSDWGG]

I checked the current failures. The Run zizmor job is failing before auditing this PR because it cannot list tags for actions-cool/issues-helper in .github/workflows/issue-close-require.yml (403 Forbidden from GitHub/git-upload-pack), so that looks unrelated to this branch. The Windows CI failure was a single playground HMR timeout (Timeout waiting for browser logs in playground/hmr/__tests__/hmr.spec.ts), also not tied to the changed 404-header code.