Show parallel test sessions in a single grid webview instead of opening new tabs

[Copilot]

Parallel test runs each spawned a new VS Code tab, causing tab explosion. This replaces per-session WebviewPanel instances with a single shared grid panel that dynamically adds iframe cells as sessions arrive.

Architecture

• Single panel: debuggerPanels: Map<string, WebviewPanel> → one gridPanel + activeSessions: Map<string, SessionData>
• Ready queue: webview posts { type: 'ready' } on load; extension queues postMessage calls until then, avoiding the async race between setting .html and sending messages
• WebSocket events forwarded to the correct iframe via _gridSessionId routing field

Grid layout

cols = Math.ceil(√n)
rows = Math.ceil(n / cols)

Sessions	Layout
1	1×1 (full-size)
2	2×1
3–4	2×2
5–6	3×2
7–9	3×3

Webview messages (extension → grid)

Type	Effect
addSession	Creates a new iframe cell with a title header
updateTitle	Updates a cell's status header (Running / Passed / Failed / Done)
removeSession	Removes a cell and recalculates layout

Replaces getWaitingHtml + getDebuggerHtml with a single getGridHtml that starts in a waiting state and manages cells dynamically without reloading existing iframes.

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.