Handle nil backtrace_locations in generate_snippet

[marcoroth]

This pull request fixes a crash in ErrorHighlight::CoreExt#generate_snippet when an exception has a backtrace but no backtrace_locations.

Exception#backtrace_locations can be nil even when backtrace is set, most commonly when an exception is rebuilt via Marshal or set_backtrace.

A real-world case: the parallel gem marshals exceptions from forked workers back to the parent process, where backtrace survives as strings but backtrace_locations is nil.

When an ArgumentError message matches one of the keyword-argument patterns (wrong number of arguments, unknown keyword, …), generate_snippet calls locs.size without checking for nil first. Displaying such an exception then raises:

error_highlight/core_ext.rb:8:in 'ErrorHighlight::CoreExt#generate_snippet':
  undefined method 'size' for nil (NoMethodError)

Since this fires inside detailed_message, it masks the original exception entirely, e.g. Rake's error display crashes and the user never sees the real error (example CI failure):

/opt/hostedtoolcache/Ruby/4.0.1/x64/lib/ruby/4.0.0/error_highlight/core_ext.rb:8:in 'ErrorHighlight::CoreExt#generate_snippet': undefined method 'size' for nil (NoMethodError)

        return "" if locs.size < 2
                         ^^^^^
	from /opt/hostedtoolcache/Ruby/4.0.1/x64/lib/ruby/4.0.0/error_highlight/core_ext.rb:43:in 'ErrorHighlight::CoreExt#detailed_message'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/rake-13.4.2/lib/rake/application.rb:264:in 'Rake::Application#display_exception_message_details'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/rake-13.4.2/lib/rake/application.rb:240:in 'Rake::Application#display_exception_details'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/rake-13.4.2/lib/rake/application.rb:231:in 'Rake::Application#display_error_message'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/rake-13.4.2/lib/rake/application.rb:218:in 'Rake::Application#standard_exception_handling'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/commands/rake/rake_command.rb:28:in 'block in Rails::Command::RakeCommand.perform'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/commands/rake/rake_command.rb:49:in 'block in Rails::Command::RakeCommand.with_rake'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/rake-13.4.2/lib/rake/rake_module.rb:59:in 'Rake.with_application'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/commands/rake/rake_command.rb:46:in 'Rails::Command::RakeCommand.with_rake'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/commands/rake/rake_command.rb:20:in 'Rails::Command::RakeCommand.perform'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/command.rb:150:in 'Rails::Command.invoke_rake'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/command.rb:67:in 'block in Rails::Command.invoke'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/command.rb:143:in 'Rails::Command.with_argv'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/command.rb:63:in 'Rails::Command.invoke'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/bundler/gems/rails-fac8810bcdec/railties/lib/rails/commands.rb:18:in '<main>'
	from /opt/hostedtoolcache/Ruby/4.0.1/x64/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require'
	from /opt/hostedtoolcache/Ruby/4.0.1/x64/lib/ruby/4.0.0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/rubyevents/rubyevents/vendor/bundle/ruby/4.0.0/gems/bootsnap-1.23.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:33:in 'Kernel#require'
	from bin/rails:4:in '<main>'

Repro:

def explode(file_path:) = nil

begin
  explode(file_path: "x", document: nil)
rescue ArgumentError => e
  Marshal.load(Marshal.dump(e)).detailed_message # => NoMethodError
end

This pull request adds the same nil guard ErrorHighlight.spot already uses in base.rb, plus a regression test using the Marshal round-trip.

[mame]

Thank you!