Fix ActiveStorage::Blob re-upload if used in with: param

[bubiche]

What this does

Fixes #665

When an ActiveStorage::Blob is passed to ask or create_user_message via with:, it is downloaded and re-uploaded as a new blob. This is because the blob gets wrapped in a RubyLLM::Attachment during content building, so it goes to the else branch in prepare_for_active_storage and convert_to_active_storage_format is called, which always creates a new IO hash.

=> Fix by updating convert_to_active_storage_format to detect ActiveStorage objects and return it directly so it can be reused.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.75%. Comparing base (4034c05) to head (77932ea).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/active_record/chat_methods.rb	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #683      +/-   ##
==========================================
+ Coverage   81.63%   81.75%   +0.12%     
==========================================
  Files         117      117              
  Lines        5522     5526       +4     
  Branches     1441     1444       +3     
==========================================
+ Hits         4508     4518      +10     
+ Misses       1014     1008       -6     

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