Every extension slash command reports "No client found for command" even though its handler runs #5412

Describe the bug

In 1.0.109, every slash command that an extension registers through joinSession({ commands }) prints ✗ Command /<name> failed: No client found for command: <name>. The extension still receives the command and its output appears right below the error, so the command works and the failure message is false.

Restarting the CLI doesn't help. It happens with every extension that registers a command, with and without arguments.

Affected version

GitHub Copilot CLI 1.0.109

Steps to reproduce the behavior

1. Create an extension with only this code:

   import { joinSession } from '@github/copilot-sdk/extension';
   const session = await joinSession({
       commands: [{ name: 'probe', description: 'probe', handler: async () => { await session.log('probe ran'); } }],
   });

2. Start an interactive session, or reload extensions, and run /probe.

3. The CLI prints ✗ Command /probe failed: No client found for command: probe, followed by probe ran.

Expected behavior

The command runs without an error.

Additional context

• macOS on Apple silicon (arm64).
• Seen with every extension that registers a command, in new, resumed, and restarted sessions.
• The process log shows the extension registering the command at startup, in a session.resume request whose commands list includes it. When the command runs, the log shows command.execute and command.completed delivered to the extension in the same millisecond, with no error logged. The error text appears only in the 1.0.109 runtime binary. The bundled copilot-sdk command dispatch is identical in 1.0.108 and 1.0.109.