docs(phpdoc): document public methods across src/

Add professional PHPDoc (summary + @param/@return) to the remaining
public methods that had none or only partial coverage: GraphController's
actions, ContributionStore's accessors, SvgRenderer's render() and
private helpers (with concrete array-shape annotations), ProbeTrait,
ProviderHealthChecker, ProviderStatus, and the Contribution entity.
Also adds the matching @return shapes on SvgRendererTest's data
providers.

src/Service/Provider/ProbeTrait.php

@@ -9,6 +9,10 @@ use Symfony\Contracts\HttpClient\Exception\TransportExceptionInterface;
 
 trait ProbeTrait
 {
+    /**
+     * Reports NotConfigured if credentials are missing, otherwise pings the provider
+     * and reports Ok or Error (with a classified error code and message).
+     */
     public function probe(): ProviderStatus
     {
         if (!$this->isConfigured()) {