[ENHANCEMENT] Add shared VS Code message capture to Playwright gallery #517

Problem

The Story Gallery now provides shared extension state, translations, React Query, tooltips, deterministic themes, and fresh provider state. Outbound VS Code messages are still captured ad hoc: Announcement.links.visual.tsx parses the plain-browser console.log fallback instead of using a typed shared test channel.

Desired behavior

Provide a reusable browser-side message capture helper for Story Gallery tests. Tests should read typed outbound vscode.postMessage payloads without starting the Extension Host, parsing console output, or duplicating setup.

Current state

PR #1426 superseded the original provider-harness scope:

• playwright/AppProviders.tsx supplies extension state, i18n, React Query, and tooltips.
• Stories can override a typed subset of extension state.
• Providers and query clients are recreated for each mounted story.
• The gallery runs in the pinned Docker/CI rendering environment.

The remaining gap is shared host-message capture.

Acceptance criteria

• A browser-side helper captures outbound VS Code messages with a typed payload shape.
• Captured messages are reset for every test or mount so state cannot leak.
• Tests can inspect messages without parsing console output or requiring the real Extension Host.
• Announcement.links.visual.tsx uses the shared capture path and still proves one host message per external link.
• The helper remains opt-in and does not obscure ordinary story setup.
• Story Gallery tests, Docker visual comparison, type checking, lint, and existing webview unit tests pass.

Constraints

• Keep real Extension Host behavior in apps/vscode-e2e; this helper only captures the browser-side message boundary.
• Preserve AppProviders as the shared provider harness rather than introducing a second wrapper.
• Keep Vitest as the primary behavioral test layer.

Related: #515, #1358, PR #1426