Simplify dynamic Shakapacker constant resolution

Replace legacy Object.const_get pattern with direct Shakapacker references
since packer_type only returns "shakapacker" now.

Changes:
- Replace Object.const_get(ReactOnRails::PackerUtils.packer_type.capitalize) with Shakapacker
- Update Manifest::MissingEntryError references
- Update DevServer references in tests
- Simplify test helper constant assignment

This removes unnecessary dynamic constant resolution that was needed
when both Webpacker and Shakapacker were supported.

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

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

Author: justin808

lib/react_on_rails/utils.rb

@@ -81,9 +81,7 @@ def self.bundle_js_file_path(bundle_name)
       if ReactOnRails::PackerUtils.using_packer? && bundle_name != "manifest.json"
         begin
           ReactOnRails::PackerUtils.bundle_js_uri_from_packer(bundle_name)
-        rescue Object.const_get(
-          ReactOnRails::PackerUtils.packer_type.capitalize
-        )::Manifest::MissingEntryError
+        rescue Shakapacker::Manifest::MissingEntryError
           # When manifest lookup fails, try multiple fallback locations:
           # 1. Environment-specific path (e.g., public/webpack/test)
           # 2. Standard Shakapacker location (public/packs)

spec/dummy/spec/helpers/react_on_rails_helper_spec.rb

@@ -3,7 +3,7 @@
 require "rails_helper"
 require "support/script_tag_utils"
 
-Packer = Object.const_get(ReactOnRails::PackerUtils.packer_type.capitalize)
+Packer = Shakapacker
 
 class PlainReactOnRailsHelper
   include ReactOnRailsHelper

spec/react_on_rails/utils_spec.rb

@@ -50,7 +50,7 @@ module ReactOnRails
     end
 
     def mock_bundle_in_manifest(bundle_name, hashed_bundle)
-      mock_manifest = instance_double(Object.const_get(ReactOnRails::PackerUtils.packer_type.capitalize)::Manifest)
+      mock_manifest = instance_double(Shakapacker::Manifest)
       allow(mock_manifest).to receive(:lookup!)
         .with(bundle_name)
         .and_return(hashed_bundle)
@@ -61,9 +61,7 @@ def mock_bundle_in_manifest(bundle_name, hashed_bundle)
     def mock_missing_manifest_entry(bundle_name)
       allow(ReactOnRails::PackerUtils.packer).to receive_message_chain("manifest.lookup!")
         .with(bundle_name)
-        .and_raise(Object.const_get(
-          ReactOnRails::PackerUtils.packer_type.capitalize
-        )::Manifest::MissingEntryError)
+        .and_raise(Shakapacker::Manifest::MissingEntryError)
     end
 
     def random_bundle_name
@@ -577,7 +575,7 @@ def mock_dev_server_running
           before do
             allow(ReactOnRails::PackerUtils.packer).to receive(:dev_server).and_return(
               instance_double(
-                Object.const_get(ReactOnRails::PackerUtils.packer_type.capitalize)::DevServer,
+                Shakapacker::DevServer,
                 running?: true,
                 protocol: "http",
                 host_with_port: "localhost:3035"