Test that the image URL's host is one spelling (part of #246)

The image URL parser must lower-case the host and refuse one holding
"@", which a URL reads as a user name and password before the host;
ErrInvalidHost is added for that refusal. A host with a port, or a ":"
with no port, must match no allowlist entry, so its request needs a
signature. All three tests fail until the fix.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -15,6 +15,7 @@ import (
 var (
 	ErrInvalidPath       = errors.New("invalid image path")
 	ErrMissingHost       = errors.New("missing source host")
+	ErrInvalidHost       = errors.New("invalid source host")
 	ErrMissingSize       = errors.New("missing size specification")
 	ErrInvalidSize       = errors.New("invalid size format")
 	ErrInvalidFormat     = errors.New("invalid or unsupported format")