fix: fallback to undiciFetch when globalThis.fetch is unavailable

[deep-innovator]

Problem

On Windows with older Node.js versions, globalThis.fetch is not available at module load time, causing the original nativeFetch definition to fail immediately.

Solution

• Keep nativeFetch as a direct request function, preserving the original signature (input, init) => Promise<Response>.
• Fall back to undiciFetch synchronously when globalThis.fetch is unavailable.
• This maintains the existing no-proxy path behavior while fixing the startup error on Windows.

Verification

• No breaking changes to the call site return nativeFetch(input, init).
• Works both with native fetch and the undiciFetch fallback.