fix(lit): align basic catalog component behaviors with angular implementation

[josemontespg]

Overview

Aligns the @a2ui/lit Basic Catalog component implementations, styling contracts, and lifecycle behaviors with the @a2ui/angular reference implementation.

Motivation & Architectural Rationale

This PR is a foundational step in our cross-framework rendering architecture:

1. Prerequisite for Universal Components (@a2ui/web_core):
   In the subsequent PRs of this stack, the Basic Catalog implementations will be extracted into unified, universal Web Components hosted in @a2ui/web_core and shared directly across @a2ui/lit, @a2ui/angular, and upcoming renderers (such as React). To ensure this migration produces zero behavioral drift or visual regressions, all renderers must first converge on a canonical DOM structure, CSS variable contract, and event model.

2. Adopting the More Complete Reference Behaviors:
   Prior to this change, minor behavioral divergence existed between renderers where the Angular implementation provided a more robust and spec-complete contract:

   • Comprehensive Form Validation (TextField): The A2UI protocol specifies validationErrors as a string array (string[]). Angular renders all accumulated validation errors for the field, whereas Lit previously truncated output to validationErrors[0].
   • Cross-Platform Layout Distribution (Column): Angular columns define width: 100% on the flex container host to ensure consistent full-width item stretching across different nesting depths and container types.
   • Natural Media Sizing (Image): Removing forced width: 100% allows images to preserve intrinsic aspect ratios and respect explicit layout constraints without unintended stretching.
   • Consistent Dialog Lifecycle & Styling (Modal): Angular's modal architecture utilizes explicit boolean state tracking (isOpen) and standard overlay CSS classes/custom properties (.a2ui-modal-overlay, .a2ui-modal-content, --a2ui-modal-backdrop-bg). Aligning Lit to this pattern unifies CSS theming tokens and guarantees consistent event propagation and backdrop click handling across frameworks.
   • Semantic Typography (Text): Wrapping variant: 'caption' in <em> ensures captions render with standardized italic typography out of the box.

Changes Made

• TextField: Renders all error strings in props.validationErrors instead of only the first index.
• Column: Adds width: 100% to :host, a2ui-basic-column styles.
• Image: Removes width: 100% from default img styling to prevent unwanted stretching.
• Modal: Adopts explicit @state() accessor isOpen = false; state tracking and .a2ui-modal-overlay / .a2ui-modal-content DOM hierarchy.
• Text: Wraps variant: 'caption' rendered markdown in <em> elements.
• Tests: Updates component unit tests and explorer integration tests to verify these aligned behaviors.

Resolves #1270

[gemini-code-assist]

Code Review

This pull request refactors the A2uiLitModal component to replace the native HTML <dialog> element with a custom overlay structure managed by Lit state, updating corresponding tests and the changelog. The review feedback focuses on enhancing the accessibility and layout of the new modal implementation, including adding keyboard navigation support to the trigger, using inset: 0 on the overlay to prevent layout issues, and adding proper ARIA roles and labels to the modal container and close button.

[josemontespg]

Don't remove this jsdoc