fix p3 request.set-authority rejecting ipv6 literal authorities

[netliomax25-code]

Repro: in WASIp3, call request.set-authority with a bracketed IPv6 literal that has no explicit port, for example [::1] or [2001:db8::1]. The call returns an error and the authority is never set.

Cause: the validation detects whether a port was supplied with authority.contains(':') before checking that the port parses. That is also true for the colons inside an IPv6 literal host, so the later "port present but not a valid number" guard fires and a perfectly valid IPv6 authority is rejected unless a port is also appended.

Fix: look for the port only after any closing ], so colons inside the IPv6 host are not mistaken for a port. A genuinely malformed port such as example.com: or example.com:abc is still rejected. The check moved into a small parse_authority helper with unit tests for the host-name, IPv4, and bracketed IPv6 forms.

[pchickey]

Thanks, I have now learned something disappointing about the http::Authority parser that I had higher expectations of. Fix much appreciated.