Fix ActiveStorage::Blob re-upload if used in with: param (#683)

## 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

- [x] Bug fix
- [ ] New feature
- [ ] Breaking change
- [ ] Documentation
- [ ] Performance improvement

## Scope check

- [x] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [x] This aligns with RubyLLM's focus on **LLM communication**
- [x] This isn't application-specific logic that belongs in user code
- [x] This benefits most users, not just my specific use case

## Quality check

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

## AI-generated code

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

## API changes

- [ ] Breaking change
- [ ] New public methods/classes
- [ ] Changed method signatures
- [x] No API changes

Co-authored-by: nebel95 <nebel95@wallwisher.com>
Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: 3 people

lib/ruby_llm/active_record/chat_methods.rb

@@ -417,11 +417,18 @@ def convert_to_active_storage_format(source)
 
         attachment = source.is_a?(RubyLLM::Attachment) ? source : RubyLLM::Attachment.new(source)
 
-        {
-          io: StringIO.new(attachment.content),
-          filename: attachment.filename,
-          content_type: attachment.mime_type
-        }
+        if attachment.active_storage?
+          case attachment.source
+          when ActiveStorage::Blob then attachment.source
+          when ActiveStorage::Attached::One, ActiveStorage::Attached::Many then attachment.source.blobs
+          end
+        else
+          {
+            io: StringIO.new(attachment.content),
+            filename: attachment.filename,
+            content_type: attachment.mime_type
+          }
+        end
       rescue StandardError => e
         RubyLLM.logger.warn "Failed to process attachment #{source}: #{e.message}"
         nil

spec/ruby_llm/active_record/acts_as_attachment_spec.rb

@@ -88,6 +88,24 @@ def attachment_io(path)
       user_message = chat.messages.find_by(role: 'user')
       expect(user_message.attachments.count).to eq(1)
     end
+
+    it 'reuses an existing ActiveStorage::Blob without re-uploading' do
+      chat = Chat.create!(model: model)
+
+      existing_blob = ActiveStorage::Blob.create_and_upload!(
+        io: attachment_io(image_path),
+        filename: 'ruby.png',
+        content_type: 'image/png'
+      )
+
+      expect do
+        chat.create_user_message('What do you see?', with: existing_blob)
+      end.not_to change(ActiveStorage::Blob, :count)
+
+      user_message = chat.messages.find_by(role: 'user')
+      expect(user_message.attachments.count).to eq(1)
+      expect(user_message.attachments.first.blob_id).to eq(existing_blob.id)
+    end
   end
 
   describe 'attachment types' do