fix(dev): apply server headers to 404 responses (fix #22543)

[sjh9714]

Summary

Fixes #22543.

server.headers was applied by built-in asset, HTML, and transform response paths, but not by the dev server fallthrough 404 path. This adds a lightweight headers middleware after CORS and host validation so POST/API-style 404 responses include configured server headers too.

Changes

• Apply configured dev server.headers before user and built-in response middlewares run.
• Add a unit regression test for a POST request that falls through to Vite's 404 handler.

Testing

• pnpm install --frozen-lockfile
• pnpm --filter=./packages/vite run build-bundle
• pnpm test-unit packages/vite/src/node/__tests__/dev.spec.ts
• pnpm exec eslint --cache --concurrency auto packages/vite/src/node/server/index.ts packages/vite/src/node/__tests__/dev.spec.ts
• pnpm --filter=./packages/vite run build-types-roll
• pnpm --filter=./packages/vite run typecheck
• git diff --check
• pnpm exec oxfmt --check packages/vite/src/node/server/index.ts packages/vite/src/node/__tests__/dev.spec.ts

Note: I used Codex while preparing this change, reviewed the final diff, and ran the listed checks locally.