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_internal_test.go

@@ -234,6 +234,49 @@ func TestParseImagePath(t *testing.T) {
 	}
 }
 
+// TestParseImagePath_LowerCasesHost verifies that the host is lower-cased, so
+// that every spelling of its letter case is the same host to the allowlist,
+// the upstream connection limits and the cache, and that the path keeps its
+// case.
+func TestParseImagePath_LowerCasesHost(t *testing.T) {
+	t.Parallel()
+
+	got, err := ParseImagePath("CDN.Example.COM/Photos/Cat.jpg/800x600.webp")
+	if err != nil {
+		t.Fatalf("ParseImagePath() error = %v", err)
+	}
+
+	t.Logf("parsed host %q, path %q", got.Host, got.Path)
+
+	if got.Host != testHostCDN {
+		t.Errorf("Host = %q, want %q", got.Host, testHostCDN)
+	}
+
+	if got.Path != "/Photos/Cat.jpg" {
+		t.Errorf("Path = %q, want %q", got.Path, "/Photos/Cat.jpg")
+	}
+}
+
+// TestParseImagePath_RefusesUserinfo verifies that a host holding "@" is
+// refused: a URL reads what comes before the "@" as a user name and password
+// and the rest as the host.
+func TestParseImagePath_RefusesUserinfo(t *testing.T) {
+	t.Parallel()
+
+	for _, input := range []string{
+		"user@cdn.example.com/photos/cat.jpg/800x600.webp",
+		"user:password@cdn.example.com/photos/cat.jpg/800x600.webp",
+	} {
+		got, err := ParseImagePath(input)
+		t.Logf("ParseImagePath(%q) = %+v, error = %v", input, got, err)
+
+		if !errors.Is(err, ErrInvalidHost) {
+			t.Errorf("ParseImagePath(%q) error = %v, want %v",
+				input, err, ErrInvalidHost)
+		}
+	}
+}
+
 func TestParsedURL_ToImageRequest(t *testing.T) {
 	t.Parallel()