fix: update default port handling in ProxyHeaderMiddleware for forwarded headers

[alukach]

What I'm changing

Updates the ProxyHeaderMiddleware._get_forwarded_url_parts() to better respect proxy forwarding headers. This ensures that applications behind reverse
proxies/load balancers can correctly determine the original client request URL.

How I did it

Rework the middleware to determine the port using the following priority order:

Port Resolution Rules

The middleware determines the port using the following priority order:

1. Initial Port Setup

   Start with protocol default: 443 for https, 80 for http

   If Host header exists:

   • With explicit port → use that port
   • Without port → keep protocol default

   If no Host header:

   • Use scope["server"] port (server's listening port)

2. Forwarding Header Override

   The middleware checks for forwarding headers in priority order:

   Standard Forwarded header (highest priority):

   • If host=domain:port → use explicit port
   • If host=domain (no port) → apply protocol default based on proto value
   • If host=domain:invalid (malformed port) → apply protocol default

   Non-standard X-Forwarded-* headers (fallback):

   • If X-Forwarded-Port is present and valid → use that port
   • If X-Forwarded-Host or X-Forwarded-Proto is present without valid port → apply protocol
     default

Examples

Protocol changes trigger port defaults:

# Original request
scheme: "http"
server: ["localhost", 8080]
headers: [(b"`x-forwarded-proto`", b"https")]

# Result
("https", "localhost", 443)  # port changed from 8080 to 443

Explicit ports are always respected:

# Original request
scheme: "http"
server: ["localhost", 8080]
headers: [(b"forwarded", b"proto=https;host=api.example.com:8443")]

# Result
("https", "api.example.com", 8443)

Invalid ports use protocol default (not original port):

# Original request
scheme: "http"
server: ["testserver", 80]
headers: [(b"forwarded", b"proto=https;host=test:not-an-integer")]

# Result
("https", "test", 443)  # uses protocol default, not 80

Server port only used when no forwarding occurs:

# Original request (no Host header, no forwarding headers)
scheme: "https"
server: ["localhost", 8000]
headers: []

# Result
("https", "localhost", 8000)  # server port preserved

Domain changes without port trigger protocol default:

# Original request
scheme: "http"
server: ["testserver", 8080]
headers: [
    (b"host", b"example.com:8080"),
    (b"`x-forwarded-proto`", b"https"),
    (b"`x-forwarded-host`", b"myproxy.com")
]

# Result
("https", "myproxy.com", 443)  # not 8080

Error Handling

• Invalid port strings in Host header → silently ignored, keeps protocol default
• Invalid port strings in forwarding headers → treated as missing, uses protocol default
• Multiple chained proxies → uses the last entry in comma-separated Forwarded header
• All ValueError exceptions during port conversion are suppressed

Related Issue(s):

• Questionable behavior with links + proxy headers #872

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[vincentsarago]

@alukach looks good to me but I didn't really follow all the logic. I'll trust your expertise on this.

Would you be able to update the changelog and do a patch release with this?