Fix structured output multi-turn conversation error

[alexey-hunter-io]

Summary

When using with_schema for structured output and continuing a conversation, the persisted Hash response was being passed directly to OpenAI instead of being serialized as a JSON string.

OpenAI API expects message content to be a string, so structured output Hashes stored in content_raw must be converted to JSON when replayed.

Changes

• Modified lib/ruby_llm/providers/openai/media.rb to convert Hash values to JSON strings in format_content
• Added test case for multi-turn conversations with structured responses

Root Cause

In format_content, when content is a Content::Raw object containing a Hash (from structured output), the Hash was returned directly:

return content.value if content.is_a?(RubyLLM::Content::Raw)

But OpenAI rejects this with: "Invalid type for 'messages[N].content': expected one of a string or array of objects, but got an object instead."

Fix

if content.is_a?(RubyLLM::Content::Raw)
  return content.value.is_a?(Hash) ? content.value.to_json : content.value
end

Fixes #497