Cast message to MessageType before creating StreamChunk in stream_broadcaster

[TheRealNeil]

Cast message to MessageType before creating StreamChunk in stream_broadcaster

Fixes #307

Summary

• Cast raw Hash messages to MessageType in stream_broadcaster so that chunk.message has the same typed interface as response.message

Root cause

During streaming, StreamChunk#message wraps the raw Hash from the provider's message stack — no type casting is applied. After generation completes, PromptResponse#messages are cast through Types::MessagesType into Common::Messages::* objects with method accessors (.content, .role).

This means chunk.message.content raises NoMethodError during streaming, while response.message.content works fine after generation. Users have to use chunk.message[:content] during streaming but response.message.content afterwards — an inconsistent API.

Fix

In stream_broadcaster, cast the message through Providers::Common::Messages::Types::MessageType when it's a Hash before passing it to StreamChunk.new. This gives chunk.message the same typed interface as response.message, so .content and .role work consistently in both streaming and non-streaming contexts.

Test plan

• Verify chunk.message.content works in on_stream callbacks
• Verify chunk.message.role works in on_stream callbacks
• Confirm non-Hash messages are passed through unchanged
• Confirm post-generation response.message.content is unaffected