fix(agent/reconnectingpty): generate rpty id before starting lifecycle (#15475)

Fixes https://github.com/coder/coder/issues/12687

There was a race condition where we would start the rpty lifecycle
before generating the ID, leading to a data race where we would try to
concurrently read and write the struct field.

agent/reconnectingpty/screen.go

@@ -67,8 +67,6 @@ func newScreen(ctx context.Context, cmd *pty.Cmd, options *Options, logger slog.
 		timeout: options.Timeout,
 	}
 
-	go rpty.lifecycle(ctx, logger)
-
 	// Socket paths are limited to around 100 characters on Linux and macOS which
 	// depending on the temporary directory can be a problem.  To give more leeway
 	// use a short ID.
@@ -80,6 +78,8 @@ func newScreen(ctx context.Context, cmd *pty.Cmd, options *Options, logger slog.
 	}
 	rpty.id = hex.EncodeToString(buf)
 
+	go rpty.lifecycle(ctx, logger)
+
 	settings := []string{
 		// Disable the startup message that appears for five seconds.
 		"startup_message off",