Fix any usages in the markdown preview sources

[mjbvz]

For #269213

[Copilot]

Pull request overview

This PR removes remaining any typings in the Markdown preview webview sources to align with the ongoing codebase cleanup in #269213 and reduce fragility from overly-broad type assertions.

Changes:

• Type acquireVsCodeApi() messaging plumbing using WebviewApi instead of any.
• Replace any event handling in style-load error tracking with DOM-safe types and null checks.
• Tighten state and DOM element typings in the preview script (state shape, active line marker, and click target traversal).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
extensions/markdown-language-features/preview-src/messaging.ts	Types the VS Code webview API parameter (WebviewApi<unknown>) to remove any from message posting.
extensions/markdown-language-features/preview-src/loading.ts	Replaces any in the style error handler with Event | string and safely reads dataset.source.
extensions/markdown-language-features/preview-src/index.ts	Types persisted state and tightens DOM click traversal typing to remove any.
extensions/markdown-language-features/preview-src/activeLineMarker.ts	Narrows the active element tracking from any to HTMLElement | undefined.

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