[BUG]: Default text renderer in the a2a-chat-canvas sample bypasses Angular HTML sanitization

# Default text renderer in the a2a-chat-canvas sample bypasses Angular HTML sanitization

Repository: https://github.com/a2ui-project/a2ui (sample project `samples/community/client/angular/projects/a2a-chat-canvas`, v0.0.1)
CWE: CWE-79 (Improper Neutralization of Input During Web Page Generation — XSS)

## Summary

The default implementation of the chat canvas's text-part renderer returns `sanitizer.bypassSecurityTrustHtml()`, and the component binds the result via `@HostBinding('innerHTML')`. Remote agent text parts are therefore rendered as raw HTML inside the host application's origin. The class does not actually render markdown; it passes the original HTML through.

## Affected code

- `samples/community/client/angular/projects/a2a-chat-canvas/src/lib/services/sanitizer-markdown-renderer-service.ts:29`
- Rendered at `default-text-part.ts:60-71` (`@HostBinding('innerHTML')`)
- Wired as the default by `markdown-renderer-service.ts:31-36` and `config.ts:103`

## Observed behavior

An agent message containing `` executes in the host page (standard stored-XSS consequence of the SafeHtml bypass).

## Context

This is community sample code (v0.0.1), not an official npm release; it is filed because the unsafe renderer is the default configuration of the sample, so applications copying the sample inherit the behavior as-is.

## Suggested remediation

Make a sanitizing pipeline the default renderer — e.g. the `@a2ui/markdown-it` path used elsewhere (markdown-it with `html: false` + DOMPurify).

Examine the files mentioned: sanitizer-markdown-renderer-service.ts, default-text-part.ts, markdown-renderer-service.ts, and config.ts. Understand how the default text renderer bypasses Angular's sanitization. The fix involves replacing the bypass with a safe pipeline, possibly using @a2ui/markdown-it with html: false and DOMPurify. Test by creating a message with an XSS payload and verifying it no longer executes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.