Remove stale TODO and dead code in AuthDigestSession.HA2

[Copilot]

Empty if (QOP == "auth-int") block with a bare // TODO and commented-out code has been dead since the file was adapted from Mono's System.Net.DigestClient. It does nothing at runtime.

• Removed the empty if block, bare // TODO, and commented-out ha2 += ... line
• Added a comment noting auth-int QOP (RFC 7616) is unsupported since it requires the entity body hash

string? HA2 (HttpURLConnection webRequest)
{
    var uri = new Uri (webRequest.URL?.ToString ()!);
    string ha2 = $"{webRequest.RequestMethod}:{uri.PathAndQuery}";
    // Note: auth-int QOP (RFC 7616) is not supported; it would require
    // hashing the entity body which is not available here.
    return HashToHexString (ha2);
}

[Copilot]

Pull request overview

Removes dead code from AuthDigestSession.HA2() (a no-op auth-int branch with TODO/commented-out code) and replaces it with an explanatory comment about auth-int being unsupported due to needing an entity-body hash.

Changes:

• Deleted the empty if (QOP == "auth-int") block and stale TODO/commented-out line.
• Added a comment documenting why auth-int isn’t supported in this implementation.