Fix X-Forwarded-For port handling

[mcrolly]

Fixes #2789

Problem

X-Forwarded-For entries can include port numbers (e.g. 1.2.3.4:1024, [::1]:8080), but ProxyHeadersMiddleware was:

1. Ignoring ports entirely — hardcoding port = 0 in scope["client"]
2. Not recognizing trusted proxies with ports — "10.0.0.1:9999" would fail ipaddress.ip_address() parsing and never match trusted hosts, so trusted proxies were treated as untrusted clients

This produced malformed client tuples like ("1.2.3.4:1024", 0).

Fix

• Added _parse_host_and_port() helper that handles IPv4 with port (1.2.3.4:8080), bracketed IPv6 with port ([::1]:8080), and bare IPv6 (::1)
• Updated __call__ to use parsed host and port instead of hardcoded port = 0
• Updated _TrustedHosts.__contains__ to strip ports before IP comparison, while preserving literal matching for unix sockets etc.

Tests

• IPv4 with port, IPv4 without port
• Bracketed IPv6 with port, bare IPv6
• Multi-proxy chain with ports on both client and trusted proxy
• Unit tests for _parse_host_and_port edge cases (invalid port string, missing brackets, etc.)
• All 252 existing tests pass, 0 regressions