Refactor Attachment#initialize to extract source handling methods

[pekopekopekopayo]

What this does

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)

API changes

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

Related issues

[FEATURE] Support for handling ActionDispatch::Http::UploadedFile in RubyLLM (filename is always nil) #468

[pekopekopekopayo]

[FEATURE] Support for handling ActionDispatch::Http::UploadedFile in RubyLLM (filename is always nil) #468