Surface HttpRequestException from AndroidMessageHandler

[Copilot]

Xamarin.Android.Net.AndroidMessageHandler reported transport/protocol failures as the legacy System.Net.WebException, contradicting the documented HttpClient.SendAsync contract (which throws HttpRequestException).

Changes

• AndroidMessageHandler: all WebException throw sites in the request path (timeout, protocol, security, IO, connect, max‑redirections, invalid redirect URI, protocol‑method setup) now throw HttpRequestException.
• Back-compat: the original WebException — including its WebExceptionStatus — is kept as the InnerException, so code migrated from classic Xamarin.Android that inspects status (and existing test helpers that walk the inner chain) keeps working. Centralized in one helper:

static HttpRequestException CreateHttpRequestException (string message, Exception? innerException, WebExceptionStatus status)
    => new HttpRequestException (message, new WebException (message, innerException, status, null));

• Tests: connecting to an unused port asserts HttpRequestException; exceeding MaxAutomaticRedirections asserts HttpRequestException with the preserved inner WebException.

Notes

• Public API surface is unchanged; only the thrown exception type differs.
• Callers catching WebException at the top level will no longer match — this is the intended behavioral change. The WebException remains reachable via ex.InnerException.