Fix render_options_spec tests for immediate_hydration

Updates render_options_spec.rb to handle immediate_hydration correctly:
- Removes immediate_hydration from configurable_options list since it's
  no longer a ReactOnRails configuration option
- Adds dedicated tests for immediate_hydration that verify:
  - Explicit true/false values work
  - Default behavior uses ProUtils.immediate_hydration_enabled?

This fixes test failures caused by trying to set immediate_hydration
on ReactOnRails.configuration which no longer exists.

Author: claude

spec/react_on_rails/react_component/render_options_spec.rb

@@ -9,7 +9,6 @@
     replay_console
     raise_on_prerender_error
     random_dom_id
-    immediate_hydration
   ].freeze
 
   def the_attrs(react_component_name: "App", options: {})
@@ -164,4 +163,39 @@ def the_attrs(react_component_name: "App", options: {})
       end
     end
   end
+
+  describe "#immediate_hydration" do
+    context "with immediate_hydration option set to true" do
+      it "returns true" do
+        options = { immediate_hydration: true }
+        attrs = the_attrs(options: options)
+
+        opts = described_class.new(**attrs)
+
+        expect(opts.immediate_hydration).to be true
+      end
+    end
+
+    context "with immediate_hydration option set to false" do
+      it "returns false" do
+        options = { immediate_hydration: false }
+        attrs = the_attrs(options: options)
+
+        opts = described_class.new(**attrs)
+
+        expect(opts.immediate_hydration).to be false
+      end
+    end
+
+    context "without immediate_hydration option" do
+      it "returns value from ProUtils.immediate_hydration_enabled?" do
+        allow(ReactOnRails::ProUtils).to receive(:immediate_hydration_enabled?).and_return(true)
+        attrs = the_attrs
+
+        opts = described_class.new(**attrs)
+
+        expect(opts.immediate_hydration).to be true
+      end
+    end
+  end
 end