Normalize existing Server-Timing values

Author: justin808

react_on_rails_pro/lib/react_on_rails_pro/concerns/stream.rb

@@ -171,7 +171,8 @@ def emit_rsc_stream_server_timing_header
       desc = "RoR Pro streamed RSC shell render (includes first renderer chunk)"
       entry = "ror_stream_shell;dur=#{duration};desc=\"#{desc}\""
       existing = response.headers["Server-Timing"]
-      response.headers["Server-Timing"] = existing.present? ? "#{existing}, #{entry}" : entry
+      entries = Array(existing).flatten.compact.reject(&:blank?) + [entry]
+      response.headers["Server-Timing"] = entries.join(", ")
     rescue StandardError => e
       # Observability must never break a real response. Swallow and log.
       begin

react_on_rails_pro/spec/react_on_rails_pro/stream_spec.rb

@@ -225,6 +225,20 @@ def build_mocked_response
       expect(server_timing).to start_with("action_total;dur=5, ror_stream_shell;dur=")
     end
 
+    it "appends to array-valued Server-Timing headers without stringifying the array" do
+      _queues, controller, stream = setup_stream_test(component_count: 0)
+      allow(stream).to receive(:write)
+      controller.response.headers["Server-Timing"] = ["cdn-cache;dur=10"]
+
+      run_stream(controller, rsc_stream_observability: true) do |_parent|
+        sleep 0.1
+      end
+
+      server_timing = controller.response.headers["Server-Timing"]
+      expect(server_timing).to start_with("cdn-cache;dur=10, ror_stream_shell;dur=")
+      expect(server_timing).not_to include("[")
+    end
+
     it "swallows Server-Timing header emission errors even when logging fails" do
       _queues, controller, _stream = setup_stream_test(component_count: 0)
       failing_headers = instance_double(Hash)