[client, proxy] cancel context before stopping engine on embedded client (#6397)

- Engine.Start takes syncMsgMux with a deferred unlock (engine.go:445) and parks in receiveSignalEvents → WaitStreamConnected (engine.go:1762), which only wakes on
  signal-stream connect or client-context cancellation.
  - When signal never connects, the 30s startup timeout fires and embed.Client.Start's rollback (embed.go:281) called client.Stop() → Engine.Stop, which blocks acquiring
  syncMsgMux (engine.go:318). The cancel() that would unpark Start was deferred until Start returned — permanent cycle. RemovePeer calls (g43/g385) then queue behind the
  lifecycle mutex.
  - Notably, embed.Client.Stop and the daemon's cleanupConnection both cancel before stopping — the startup rollback was the only path that didn't.
  - Engine.Start takes syncMsgMux with a deferred unlock (engine.go:445) and parks in receiveSignalEvents → WaitStreamConnected (engine.go:1762), which only wakes on
  signal-stream connect or client-context cancellation.
  - When signal never connects, the 30s startup timeout fires and embed.Client.Start's rollback (embed.go:281) called client.Stop() → Engine.Stop, which blocks acquiring
  syncMsgMux (engine.go:318). The cancel() that would unpark Start was deferred until Start returned — permanent cycle. RemovePeer calls (g43/g385) then queue behind the
  lifecycle mutex.
  - Notably, embed.Client.Stop and the daemon's cleanupConnection both cancel before stopping — the startup rollback was the only path that didn't.

client/embed/embed.go

@@ -279,6 +279,10 @@ func (c *Client) Start(startCtx context.Context) error {
 
 	select {
 	case <-startCtx.Done():
+		// Cancel the client context before stopping: Engine.Start blocks on the
+		// signal stream while holding the engine mutex and only unblocks on
+		// cancellation. Stopping first would deadlock on that mutex.
+		cancel()
 		if stopErr := client.Stop(); stopErr != nil {
 			return fmt.Errorf("stop error after context done. Stop error: %w. Context done: %w", stopErr, startCtx.Err())
 		}