Fix Bun default import handling for Babel helpers

[gaoflow]

Summary

• Handle Babel CommonJS default imports when runtimes already return the default export directly
• Add coverage for both wrapped and already-unwrapped default import shapes
• Add a patch change file for @react-router/dev

Fixes #15209.

AI assistance was used under my direction.

Testing

• pnpm test packages/react-router-dev/vite/babel-test.ts packages/react-router-dev/vite/remove-exports-test.ts
• bun --bun -e "import { generate, parse } from './packages/react-router-dev/vite/babel.ts'; const ast = parse('const x = 1', { sourceType: 'module' }); console.log(typeof generate); console.log(generate(ast).code);"
• pnpm run --filter @react-router/dev build
• pnpm run changes:validate
• pnpm run --filter react-router build
• pnpm test packages/react-router-dev/
• pnpm run --filter @react-router/dev typecheck
• pnpm exec prettier --check packages/react-router-dev/vite/babel.ts packages/react-router-dev/vite/babel-test.ts packages/react-router-dev/.changes/patch.bun-babel-default-imports.md
• pnpm exec eslint packages/react-router-dev/vite/babel.ts packages/react-router-dev/vite/babel-test.ts
• bun --bun ../../packages/react-router-dev/dist/cli/index.js typegen from playground/framework

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