build: remove leftover Bun polyfills from the npm bundle

[betegon]

Problem

PR #1421 fixes the globalThis.Bun side effect, but Bun was already fully removed from the CLI. node-polyfills.ts, its esbuild injection, and the related tests are leftover migration code in a Node-only project.

Scope

• Remove node-polyfills.ts and its bundle.ts inject entries.
• Remove the polyfill-specific tests and clean up stale Bun comments.
• Verify the npm CJS/ESM imports and CLI smoke tests on supported Node versions.