Replace remaining BlazorChatMessage.Content usages with Text in samples and README

[Copilot]

This PR completes the migration away from obsolete BlazorChatMessage.Content in the Blazor chat examples by updating the remaining template usages and aligned README snippets. It keeps unrelated Content APIs (for example MessageSentEventArgs.Content) untouched.

• Source example updates

  • Chat-CustomMessage.razor: @context.Content → @context.Text
  • Chat-Markdown.razor: @ToHtml(context.Content) → @ToHtml(context.Text)

• README snippet alignment

  • Updated the custom message snippet to use @context.Text
  • Updated the markdown response snippet to use @ToHtml(context.Text)

• Scope guardrails

  • No changes to unrelated Content usages such as:
    • args.Content (MessageSentEventArgs.Content)
    • ContentFormat and documentation links referencing other APIs

<div class="my-chat-content">
    @ToHtml(context.Text)
</div>