Fix version check failing on non-200 status responses

When the engine returns 401 (auth required) or other non-200 responses,
the version check was parsing the error body, getting an empty version
string, and fatally exiting. Now skips the check on non-200 responses
and lets the actual API call surface the real error.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/internal/api/client.go

@@ -94,6 +94,10 @@ func (c *Client) checkEngineVersion() {
 	}
 	defer resp.Body.Close()
 
+	if resp.StatusCode != http.StatusOK {
+		return // auth error or other issue — let the actual request surface it
+	}
+
 	var status struct {
 		Version string `json:"version"`
 	}