fix: add explicit default auth challenge handling to SessionDelegate

[liefran-sim]

Summary

• Adds urlSession(_:didReceive challenge:completionHandler:) to SessionDelegate with .performDefaultHandling
• Fixes SSE connections silently failing when going through MITM proxies (mitmproxy, Charles Proxy, etc.)

Closes #50

Problem

When using EventSource through an MITM proxy with a trusted CA certificate installed at the OS level, SSE connections fail silently. The request never reaches the proxy and no error events are emitted.

SessionDelegate implements URLSessionDataDelegate but does not implement the authentication challenge delegate method. While URLSession should fall back to default system trust evaluation, in practice this can cause connections to be silently rejected when a custom delegate is present without explicit challenge handling.

Fix

Add explicit .performDefaultHandling to SessionDelegate:

func urlSession(
    _ session: URLSession,
    didReceive challenge: URLAuthenticationChallenge,
    completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void
) {
    completionHandler(.performDefaultHandling, nil)
}

This explicitly delegates to the system's default trust evaluation, which correctly respects user-installed CA certificates. This has no effect on normal (non-proxy) connections — it simply makes the default behavior explicit rather than implicit.

[Recouse]

Hi,
Thanks for the PR!

I’ll accept it, but in the future, it would be good to support custom authentication, similar to the approach used in #47.