Components.AI: Correct default plain-text rendering

### Is your feature request related to a problem? Please describe the problem.

Default text rendering does not fully define whitespace and newline behavior.

### Describe the solution you'd like

Render model text as safely escaped plain text with correct whitespace and newline preservation. Explicitly do not interpret markdown or HTML in the core default renderer.

### Additional context

Leaf task for #68227 requirement J1 and decision D1.

Read the parent issue #68227, especially requirement J1 and decision D1, before locating the core default renderer. Verify the expected whitespace and newline behavior, then ensure model text is safely escaped as plain text without interpreting Markdown or HTML. Done means the default rendering behavior matches those requirements.

Written by the indexing model from the issue text.