Add --test-viessmann flag for end-to-end API validation

Adds GetDHWTemperature() to the Viessmann client (GET on the DHW feature
endpoint) and a --test-viessmann CLI flag that runs three stages without
starting the EMS control loop:

  Stage 1: read current DHW setpoint (confirms auth + endpoint path)
  Stage 2: write same value back, read-back to confirm round-trip
  Stage 3: write +1°C, read-back to confirm, restore original

The +1°C delta is below the heat pump's 5°C hysteresis so the compressor
will not fire, but the change is visible in the Viessmann app for visual
confirmation before trusting the WW boost logic in production.

Usage: ./ems --config /etc/ems/ems-config.yaml --test-viessmann

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

internal/viessmann/client.go

@@ -137,6 +137,52 @@ func (c *Client) featureURL(feature string) string {
 	)
 }
 
+// dhwFeatureResponse is the minimal structure returned by GET on the DHW temperature feature.
+type dhwFeatureResponse struct {
+	Data struct {
+		Properties struct {
+			Value struct {
+				Value float64 `json:"value"`
+			} `json:"value"`
+		} `json:"properties"`
+	} `json:"data"`
+}
+
+// GetDHWTemperature reads the current domestic hot water target temperature from the Viessmann API.
+func (c *Client) GetDHWTemperature(ctx context.Context) (float64, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.ensureToken(ctx); err != nil {
+		return 0, fmt.Errorf("ensuring token: %w", err)
+	}
+
+	featureURL := c.featureURL("heating.dhw.temperature.main")
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, featureURL, nil)
+	if err != nil {
+		return 0, fmt.Errorf("creating request: %w", err)
+	}
+	req.Header.Set("Authorization", "Bearer "+c.token.AccessToken)
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return 0, fmt.Errorf("API call: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return 0, fmt.Errorf("Viessmann API returned %d", resp.StatusCode)
+	}
+
+	var result dhwFeatureResponse
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return 0, fmt.Errorf("decoding response: %w", err)
+	}
+
+	return result.Data.Properties.Value.Value, nil
+}
+
 // SetDHWTemperature sets the domestic hot water target temperature via the Viessmann API.
 func (c *Client) SetDHWTemperature(ctx context.Context, tempC float64) error {
 	c.mu.Lock()