Fix server bundle path resolution in test environments (#1797)

- Fix fallback logic when manifest lookup fails to check multiple locations
- Try environment-specific path, standard location (public/packs), then generated assets path
- Return first path where bundle file actually exists
- Maintains backward compatibility with original behavior as final fallback
- Add comprehensive test cases to prevent regression

Fixes issue where server rendering failed in test environments when bundles
were in standard Shakapacker location but manifest lookup pointed to
environment-specific directory.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    react_on_rails (16.0.1.rc.0)
+    react_on_rails (16.0.1.rc.2)
       addressable
       connection_pool
       execjs (~> 2.5)

lib/react_on_rails/utils.rb

@@ -84,10 +84,24 @@ def self.bundle_js_file_path(bundle_name)
         rescue Object.const_get(
           ReactOnRails::PackerUtils.packer_type.capitalize
         )::Manifest::MissingEntryError
-          File.expand_path(
-            File.join(ReactOnRails::PackerUtils.packer_public_output_path,
-                      bundle_name)
-          )
+          # When manifest lookup fails, try multiple fallback locations:
+          # 1. Environment-specific path (e.g., public/webpack/test)
+          # 2. Standard Shakapacker location (public/packs)
+          # 3. Generated assets path (for legacy setups)
+          fallback_locations = [
+            File.join(ReactOnRails::PackerUtils.packer_public_output_path, bundle_name),
+            File.join("public", "packs", bundle_name),
+            File.join(generated_assets_full_path, bundle_name)
+          ].uniq
+
+          # Return the first location where the bundle file actually exists
+          fallback_locations.each do |path|
+            expanded_path = File.expand_path(path)
+            return expanded_path if File.exist?(expanded_path)
+          end
+
+          # If none exist, return the environment-specific path (original behavior)
+          File.expand_path(fallback_locations.first)
         end
       else
         # Default to the non-hashed name in the specified output directory, which, for legacy

spec/react_on_rails/utils_spec.rb

@@ -134,6 +134,32 @@ def mock_dev_server_running
 
               it { is_expected.to eq("#{packer_public_output_path}/manifest.json") }
             end
+
+            context "when file not in manifest and fallback to standard location" do
+              before do
+                mock_missing_manifest_entry("webpack-bundle.js")
+              end
+
+              let(:standard_path) { File.expand_path(File.join("public", "packs", "webpack-bundle.js")) }
+              let(:env_specific_path) { File.join(packer_public_output_path, "webpack-bundle.js") }
+
+              it "returns standard path when bundle exists there" do
+                allow(File).to receive(:exist?).and_call_original
+                allow(File).to receive(:exist?).with(File.expand_path(env_specific_path)).and_return(false)
+                allow(File).to receive(:exist?).with(standard_path).and_return(true)
+
+                result = described_class.bundle_js_file_path("webpack-bundle.js")
+                expect(result).to eq(standard_path)
+              end
+
+              it "returns environment-specific path when no bundle exists anywhere" do
+                allow(File).to receive(:exist?).and_call_original
+                allow(File).to receive(:exist?).and_return(false)
+
+                result = described_class.bundle_js_file_path("webpack-bundle.js")
+                expect(result).to eq(File.expand_path(env_specific_path))
+              end
+            end
           end
         end
 
@@ -189,6 +215,42 @@ def mock_dev_server_running
 
               expect(path).to end_with("public/webpack/development/#{server_bundle_name}")
             end
+
+            context "with bundle file existing in standard location but not environment-specific location" do
+              it "returns the standard location path" do
+                server_bundle_name = "server-bundle.js"
+                mock_bundle_configs(server_bundle_name: server_bundle_name)
+                mock_missing_manifest_entry(server_bundle_name)
+
+                # Mock File.exist? to return false for environment-specific path but true for standard path
+                standard_path = File.expand_path(File.join("public", "packs", server_bundle_name))
+                env_specific_path = File.join(packer_public_output_path, server_bundle_name)
+
+                allow(File).to receive(:exist?).and_call_original
+                allow(File).to receive(:exist?).with(File.expand_path(env_specific_path)).and_return(false)
+                allow(File).to receive(:exist?).with(standard_path).and_return(true)
+
+                path = described_class.server_bundle_js_file_path
+
+                expect(path).to eq(standard_path)
+              end
+            end
+
+            context "with bundle file not existing in any fallback location" do
+              it "returns the environment-specific path as final fallback" do
+                server_bundle_name = "server-bundle.js"
+                mock_bundle_configs(server_bundle_name: server_bundle_name)
+                mock_missing_manifest_entry(server_bundle_name)
+
+                # Mock File.exist? to return false for all paths
+                allow(File).to receive(:exist?).and_call_original
+                allow(File).to receive(:exist?).and_return(false)
+
+                path = described_class.server_bundle_js_file_path
+
+                expect(path).to end_with("public/webpack/development/#{server_bundle_name}")
+              end
+            end
           end
 
           context "with server file in the manifest, used for client", packer_type.to_sym do