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 81.70%. Comparing base (a5f3dbd) to head (50fbacb).
⚠️ Report is 2 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   81.67%   81.70%   +0.03%     
==========================================
  Files         169      169              
  Lines        7641     7676      +35     
  Branches     1269     1277       +8     
==========================================
+ Hits         6241     6272      +31     
+ Misses        891      888       -3     
- Partials      509      516       +7     

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