session/new deep links reject valid EMU usernames with underscores

### Short summary

New-session deep links reject a connected repository when the Enterprise Managed User owner contains an underscore.

### Affected version or release

GitHub Copilot app v1.1.10 on Windows

### Installation context

Local Windows app signed in with the Enterprise Managed User alias_microsoft; private repository alias_microsoft/REPO is already connected as a local project.

### What happened?

The documented session deep link rejects a valid repository owned by an Enterprise Managed User whose username contains an underscore.

This link fails:

ghapp://session/new?repo=alias_microsoft%2FREPO&mode=interactive&prompt=hello%20world

The app shows:

Invalid repo format: "alias_microsoft/REPO". Expected "owner/repo".

The path-style form also fails:

ghapp://session/new/alias_microsoft/REPO?mode=interactive

It shows:

Missing required 'repo' parameter (e.g. ?repo=owner/repo).

The repository exists, the signed-in account has access, and ghapp://repo/alias_microsoft/REPO opens the connected project successfully. Encoding the underscore as %5F does not change the failure.

### Steps to reproduce

1. Sign in to the GitHub Copilot app with an Enterprise Managed User whose username contains an underscore.
2. Connect a repository owned by that account.
3. Open ghapp://session/new?repo=alias_microsoft%2FREPO&mode=interactive&prompt=hello%20world.
4. Observe the Invalid repo format error.
5. Open ghapp://repo/alias_microsoft/REPO and confirm that the same repository opens normally.

### Expected behavior

The documented session/new link should accept the valid OWNER/REPO value, open the confirmation flow, and create one session with the supplied mode and prompt. EMU usernames with an underscore should pass the same repository validation used by the working repo deep link.

### Additional context

GitHub documents both session/new?repo=OWNER%2FREPO and session/new/OWNER/REPO. Managed-user documentation also states that GitHub appends an underscore and enterprise shortcode to EMU usernames.

Current workaround: open ghapp://repo/OWNER/REPO, then create the session manually from that project composer.

The app log records the same decoded value shown in the UI error: alias_microsoft/REPO. No secret or credential is involved.