fix(tui): Preserve Rich Text styling in collapsible content for diff highlighting

[sjathin]

• A human has tested these changes.

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Why

The TUI was stripping Rich Text styling from SDK event visualizations by converting Text objects to plain strings via str(). The SDK's FileEditorObservation.visualize already generates colored diff output (red for removals, green for additions) through its visualize_diff() function, but this styling was lost before reaching the user. All collapsible content rendered as unstyled plain text.

Summary

• Return event.visualize Text object directly from _build_observation_content() instead of str(event.visualize), preserving Rich styling (colors, bold, dim)
• Pass Text objects through _create_titled_collapsible and _create_event_collapsible instead of escaping and stringifying
• Widen _update_widget_in_ui signature to accept str | Text
• Add 4 tests verifying Text objects and style spans are preserved through the rendering pipeline

Issue Number

Closes #308

Video/Screenshots

Before

After

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

• This is Phase 1 of [CLI]: Add rich diff viewer inside the TUI #308. The SDK already had full diff visualization support — the TUI was just discarding the styling. No new diff rendering logic was needed.
• Phase 2 (separate PR): side-by-side diff view with a custom Textual widget
• The _escape_rich_markup method is still used and correct for title strings (which use Rich markup syntax). It was only incorrectly applied to collapsible content (which uses Static(markup=False) and renders Text objects directly).

[all-hands-bot]

🟢 Good taste — Elegant simplification that preserves Rich Text styling.

Key insight: This change eliminates lossy string conversion and lets Rich Text objects flow through the rendering pipeline intact, preserving diff colors and other styling that the SDK already generates.

Risk Assessment: 🟢 LOW — Additive change (accepts str | Text instead of just str), all 74 tests pass, and Static widgets already support both types.