Fix Viessmann API hostname and harden test propagation waits

- Update apiBase: api.viessmann.com → api.viessmann-climatesolutions.com
  (Viessmann renamed their IoT API host; old domain is NXDOMAIN)
- Extend --test-viessmann context to 120s
- Add 30s propagation wait before read-back in both Stage 3 and Restore:
  cloud accepts writes immediately but gateway polls on its own schedule,
  so GET returns the last confirmed device value with ~30s lag

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/viessmann/client.go

@@ -17,7 +17,7 @@ import (
 
 const (
 	tokenEndpoint = "https://iam.viessmann-climatesolutions.com/idp/v3/token"
-	apiBase       = "https://api.viessmann.com/iot/v2"
+	apiBase       = "https://api.viessmann-climatesolutions.com/iot/v2"
 )
 
 // tokenFile mirrors the JSON structure stored on disk.