markdown-stream: Images should be responsive

Images currently aren't constrained with `max-width: 100%;` <img width="1025" height="427" alt="Image" src="https://github.com/user-attachments/assets/886c8f5d-813b-4a2a-a135-a777eebb58ee" /> ## Example chat transcript ````markdown ## User What's the latest posts from Posit (posit.co/blog) ## Assistant Here are the latest posts from the Posit blog with direct links and images: ## Latest Posts (December 2025) 1. **[Containerization in Posit Connect: Stable, Repeatable Deployments – No Dockerfile Required](https://posit.co/blog/containerization-in-posit-connect-stable-repeatable-deployments/)** (Dec 16)  - By Kara Woo - Posit Connect will soon support containerization for building container images of deployed content - Enables stable, repeatable deployments without requiring Dockerfiles 2. **[Local models are not there (yet)](https://posit.co/blog/local-models-are-not-there-yet/)** (Dec 16)  - By Simon Couch & Sara Altman - Analysis showing that LLMs capable of running on laptops aren't yet powerful enough to effectively drive coding agents 3. **[Posit Workbench for Financial Services and Insurance](https://posit.co/blog/posit-workbench-for-financial-services-and-insurance/)** (Dec 15)  - By Posit Team - Highlighting how data science teams use Posit Workbench throughout the analytical lifecycle - Focus on financial services and insurance industries ````