Add integration tests for server page routing

These tests exercise the full pipeline: parsing source files, then
resolving page URLs through the server's route method. They verify
that text pages (.md, .rdoc), class pages, the index, and 404s all
work correctly. The text page tests would have caught the
update_parser_of_file key mismatch bug.

Author: st0012

test/rdoc/rdoc_server_test.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+require_relative 'support/test_case'
+
+class RDocServerTest < RDoc::TestCase
+
+  def setup
+    super
+
+    @dir = Dir.mktmpdir("test_rdoc_server_")
+
+    File.write File.join(@dir, "PAGE.md"), "# A Page\n\nSome content.\n"
+    File.write File.join(@dir, "NOTES.rdoc"), "= Notes\n\nSome notes.\n"
+    File.write File.join(@dir, "example.rb"), "# A class\nclass Example; end\n"
+
+    @options.files = [@dir]
+    @options.op_dir = File.join(@dir, "_site")
+    @options.root = Pathname(@dir)
+    @options.verbosity = 0
+    @options.finish
+
+    @rdoc.options = @options
+    @rdoc.store = RDoc::Store.new(@options)
+
+    capture_output do
+      @rdoc.parse_files @options.files
+    end
+    @rdoc.store.complete @options.visibility
+
+    @server = RDoc::Server.new(@rdoc, 0)
+  end
+
+  def teardown
+    FileUtils.rm_rf @dir
+    super
+  end
+
+  def test_route_serves_text_page
+    status, content_type, body = @server.send(:route, '/PAGE_md.html')
+
+    assert_equal 200, status
+    assert_equal 'text/html', content_type
+    assert_include body, 'A Page'
+  end
+
+  def test_route_serves_rdoc_text_page
+    status, content_type, body = @server.send(:route, '/NOTES_rdoc.html')
+
+    assert_equal 200, status
+    assert_equal 'text/html', content_type
+    assert_include body, 'Notes'
+  end
+
+  def test_route_serves_class_page
+    status, content_type, body = @server.send(:route, '/Example.html')
+
+    assert_equal 200, status
+    assert_equal 'text/html', content_type
+    assert_include body, 'Example'
+  end
+
+  def test_route_serves_index
+    status, content_type, _body = @server.send(:route, '/')
+
+    assert_equal 200, status
+    assert_equal 'text/html', content_type
+  end
+
+  def test_route_returns_404_for_missing_page
+    status, content_type, _body = @server.send(:route, '/nonexistent.html')
+
+    assert_equal 404, status
+    assert_equal 'text/html', content_type
+  end
+end