feat(web_core): add universal markdown fallback in text component

[josemontespg]

Description

Implements universal markdown fallback support in @a2ui/web_core basic catalog text component.

Key Changes

1. Dynamic Optional Markdown Renderer:

   • Added optional dynamic import('@a2ui/markdown-it') fallback when no custom markdownRenderer is provided via @lit/context or component properties.
   • If @a2ui/markdown-it is available in consumer bundles, text components asynchronously render rich Markdown into contextual DOM fragments.
   • If @a2ui/markdown-it is not installed or fails to load, text components gracefully fallback to plain text rendering without runtime errors or console spam.

2. Race Condition Prevention:

   • Added stale-render guards (if (value !== this.lastValue) return;) in MarkdownDirective for both custom renderers and dynamic default renderer resolutions to prevent out-of-order renders during rapid text updates (e.g. streaming LLM responses).

3. Peer Dependency & Documentation:

   • Registered @a2ui/markdown-it in peerDependencies (marked optional) and devDependencies in web_core/package.json.
   • Added changelog entry in renderers/web_core/CHANGELOG.md.

4. Testing:

   • Added unit test suite in renderers/web_core/src/v0_9/basic_catalog/directives/markdown.test.ts verifying fallback rendering, custom renderers, and out-of-order promise resolution protection.

Pre-launch Checklist

• I read the [Contributors Guide].
• I read the [Style Guide].
• My code changes (if any) have tests.

[gemini-code-assist]

Code Review

This pull request introduces @a2ui/markdown-it as an optional peer dependency to @a2ui/web_core to enable automatic Markdown rendering for text components, alongside adding unit tests for the MarkdownDirective. Feedback on these changes suggests improving the directive's robustness against race conditions by using an auto-incrementing renderId counter instead of simple value comparison. Additionally, the reviewer recommends caching the resolved renderer state synchronously to avoid unnecessary microtask overhead on subsequent renders, and removing the console warning when the optional peer dependency is not installed to prevent console clutter.