Expose persisted assistant message during streaming

[goianiense]

Summary

• Add a public attr_reader :message on acts_as_chat models that exposes the assistant record currently being persisted during complete/ask (including across tool-flow turns).
• Lets streaming blocks broadcast against the stable in-memory record without relying on messages.last, before_message capture, or private ivar access.
• Update the chat UI generator job template and streaming docs to use the new reader.

Closes #814

Usage

chat.complete do |chunk|
  chat.message.broadcast_append_chunk(chunk.content) if chunk.content.present?
end

Test plan

• Specs for chat.message nil before completion and set afterwards
• Specs for chat.message available inside the streaming block
• Specs for reassignment on tool-flow assistant turns
• Generator job expectations updated and passing
• CI green on this PR