fix(ci): only enable SimpleCov when COVERAGE=true

SimpleCov was causing exit code 1 even with 0 test failures due to
Cobertura formatter XML parsing issues. Now SimpleCov only runs when
COVERAGE environment variable is set.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: yhk1038

spec/spec_helper.rb

@@ -4,23 +4,31 @@
 require "fileutils"
 
 # SimpleCov configuration for code coverage tracking
-require "simplecov"
-require "simplecov-cobertura"
-
-SimpleCov.formatters = SimpleCov::Formatter::MultiFormatter.new([
-                                                                  SimpleCov::Formatter::HTMLFormatter,
-                                                                  SimpleCov::Formatter::CoberturaFormatter,
-                                                                ])
-
-SimpleCov.start do
-  add_filter "/spec/"
-  add_filter "/bin/"
-
-  add_group "Libraries", "/lib/"
-  add_group "CLI", "/lib/t_ruby/cli"
-  add_group "Compiler", "/lib/t_ruby/compiler"
-  add_group "Config", "/lib/t_ruby/config"
-  add_group "Version", "/lib/t_ruby/version"
+if ENV["COVERAGE"]
+  require "simplecov"
+
+  formatters = [SimpleCov::Formatter::HTMLFormatter]
+
+  # Only add Cobertura formatter if it loads successfully
+  begin
+    require "simplecov-cobertura"
+    formatters << SimpleCov::Formatter::CoberturaFormatter
+  rescue LoadError
+    # simplecov-cobertura not available
+  end
+
+  SimpleCov.formatters = SimpleCov::Formatter::MultiFormatter.new(formatters)
+
+  SimpleCov.start do
+    add_filter "/spec/"
+    add_filter "/bin/"
+
+    add_group "Libraries", "/lib/"
+    add_group "CLI", "/lib/t_ruby/cli"
+    add_group "Compiler", "/lib/t_ruby/compiler"
+    add_group "Config", "/lib/t_ruby/config"
+    add_group "Version", "/lib/t_ruby/version"
+  end
 end
 
 # Load the t-ruby library