fix(next-mdx): report unresolved plugin paths

[mturac]

Summary

Closes #94118.

• forward MDX plugin resolution failures to the webpack loader callback
• include the unresolved plugin name in the surfaced build error
• add coverage for missing string plugin paths without changing the existing happy-path fixture

Tests

• pnpm build-all
• node --check packages/next-mdx/mdx-js-loader.js
• pnpm prettier --check packages/next-mdx/mdx-js-loader.js test/e2e/app-dir/mdx/mdx.test.ts
• pnpm --filter @next/mdx pack-for-isolated-tests
• git diff --check
• HEADLESS=true pnpm test-start-webpack test/e2e/app-dir/mdx/mdx.test.ts -t "should report unresolved string plugin paths during build"
• HEADLESS=true pnpm test-start-webpack test/e2e/app-dir/mdx/mdx.test.ts
• HEADLESS=true pnpm test-start-turbo test/e2e/app-dir/mdx/mdx.test.ts