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/ProviderStatus.php

@@ -4,8 +4,15 @@ declare(strict_types=1);
 
 namespace GitContributionGraph\Service\Provider;
 
+/** Result of probing a single provider, as reported by the /health endpoint. */
 final class ProviderStatus
 {
+    /**
+     * @param string              $name    provider identifier, e.g. "github"
+     * @param ProviderStatusType  $status  overall outcome of the probe
+     * @param ?ProviderErrorCode  $error   classified error code, set only when $status is Error
+     * @param ?string             $message human-readable error detail, set only when $status is Error
+     */
     public function __construct(
         public readonly string $name,
         public readonly ProviderStatusType $status,
@@ -13,7 +20,11 @@ final class ProviderStatus
         public readonly ?string $message = null,
     ) {}
 
-    /** @return array<string, string> */
+    /**
+     * Converts to the array shape used in the /health JSON response, omitting error/message when unset.
+     *
+     * @return array<string, string>
+     */
     public function toArray(): array
     {
         $data = ['status' => $this->status->value];