Windows: 'Select an app to open' (OpenWith) dialog on every message when profile path contains a space #5299

Problem
On Windows, every message sent in the GitHub Copilot app triggers the Windows "Select an app to open 'X'" dialog (OpenWith). X is only the first word of my Windows profile folder name, which contains a space (C:\Users\First Last). The dialog appears hundreds of times. It does not happen with Microsoft Copilot.

Evidence (process watcher, 30 Oct 2026)

• 14:08:25 OpenWith.exe -Embedding started 3 times, parent svchost.exe (Windows launches it when an open request has no registered handler).
• Same instant: copilot-runtime.exe spawned pwsh.exe -NoProfile -NoLogo -NonInteractive -Command ... (the shell tool).
• Copilot runtime log has no open/shell/launch entries around the event.
• No instructions, MCP config, agents or plugins reference the word; the only matches are the profile path.

Likely cause
An unquoted path (or openPath/ShellExecute-style call) containing a space is split at the space, so Windows is asked to open only the first word, which has no file association.

Expected
No open request, or the path is quoted so it resolves correctly.

Repro
Windows user profile with a space in the name, open the Copilot app, send any message.