[relay] Use instanceURL instead of Exposed address. (#4905)

Replaces string-based exposed address handling with URL-based InstanceURL() (type url.URL) across relay/server and relay/healthcheck; adds SchemeREL/SchemeRELS constants; updates getInstanceURL to return *url.URL with scheme and TLS validation; adjusts WS dialing and health-check logic to use URL fields.

relay/server/url_test.go

@@ -0,0 +1,39 @@
+package server
+
+import "testing"
+
+func TestGetInstanceURL(t *testing.T) {
+	tests := []struct {
+		name           string
+		exposedAddress string
+		tlsSupported   bool
+		expectedURL    string
+		expectError    bool
+	}{
+		{"Valid address with TLS", "example.com", true, "rels://example.com", false},
+		{"Valid address without TLS", "example.com", false, "rel://example.com", false},
+		{"Valid address with scheme", "rel://example.com", false, "rel://example.com", false},
+		{"Invalid address with non TLS scheme and TLS true", "rel://example.com", true, "", true},
+		{"Valid address with TLS scheme", "rels://example.com", true, "rels://example.com", false},
+		{"Valid address with TLS scheme and TLS false", "rels://example.com", false, "rels://example.com", false},
+		{"Valid address with TLS scheme and custom port", "rels://example.com:9300", true, "rels://example.com:9300", false},
+		{"Invalid address with multiple schemes", "rel://rels://example.com", false, "", true},
+		{"Invalid address with unsupported scheme", "http://example.com", false, "", true},
+		{"Invalid address format", "://example.com", false, "", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			url, err := getInstanceURL(tt.exposedAddress, tt.tlsSupported)
+			if (err != nil) != tt.expectError {
+				t.Errorf("expected error: %v, got: %v", tt.expectError, err)
+			}
+			if !tt.expectError && url != nil && url.String() != tt.expectedURL {
+				t.Errorf("expected URL: %s, got: %s", tt.expectedURL, url.String())
+			}
+			if tt.expectError && url != nil {
+				t.Errorf("expected nil URL on error, got: %s", url.String())
+			}
+		})
+	}
+}