Fix Apertus parser: convert reasoning_content to content.blocks format
The Apertus template expects reasoning in a blocks format
(message.content.blocks with {"type": "thoughts", "text": "..."})
instead of the standard reasoning_content field.
This commit adds conversion logic to transform reasoning_content into
the content.blocks format before applying the template, enabling proper
reasoning support for Apertus while maintaining compatibility with
the standard API.