Support Action Text embedded attachments

[crmne]

Summary

• extract embedded Action Text Active Storage attachables into RubyLLM::Content attachments
• share Action Text content extraction between current and legacy acts_as message paths
• replace stubbed Action Text specs with real has_rich_text models and real embedded blobs
• add the Action Text rich text table to the dummy app schema/migration

Testing

• bundle exec rspec spec/ruby_llm/active_record/acts_as_action_text_spec.rb
• bundle exec rspec spec/ruby_llm/active_record
• bundle exec rubocop lib/ruby_llm/active_record/content_extraction.rb lib/ruby_llm/active_record/message_methods.rb lib/ruby_llm/active_record/acts_as_legacy.rb spec/ruby_llm/active_record/acts_as_action_text_spec.rb spec/dummy/db/migrate/20260506120000_create_action_text_tables.action_text.rb

Follow-up to #365.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.94%. Comparing base (80fe294) to head (e5075a3).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/active_record/attachment_helpers.rb	90.90%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #760      +/-   ##
==========================================
+ Coverage   82.88%   82.94%   +0.05%     
==========================================
  Files         142      142              
  Lines        6574     6538      -36     
  Branches     1148     1145       -3     
==========================================
- Hits         5449     5423      -26     
+ Misses        663      653      -10     
  Partials      462      462              

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