Add csv gem dependency for Ruby 3.4+ support

Ruby 3.4 removed csv from the default gems, requiring it to be
explicitly added as a dependency. The gem is used in three places:
- lib/jsonapi/basic_resource.rb:734 - parsing filter values
- lib/jsonapi/request.rb:353 - parsing included resources
- lib/jsonapi/request.rb:415 - parsing sorts

Without this dependency, Ruby 3.4 fails with:
  cannot load such file -- csv (LoadError)

Adding csv as a runtime dependency is safe for all Ruby versions
as it's available in the standard library for older versions.

Fixes Ruby 3.4 CI failures.

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

Author: takaokouji

jsonapi-resources.gemspec

@@ -30,4 +30,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'activerecord', '>= 5.1'
   spec.add_dependency 'railties', '>= 5.1'
   spec.add_dependency 'concurrent-ruby'
+  spec.add_dependency 'csv' # Required for Ruby 3.4+ (no longer a default gem)
 end