refactor(provider): split fetch() into startFetch()/resolveFetch()

Splits the provider contract into a non-blocking startFetch() that
fires the HTTP request(s) and a resolveFetch() that reads the result,
so callers can fire every provider's request before blocking on any
of them. Symfony HttpClient is already async under the hood -
request() doesn't block until you read the response - so this needs
no new dependency.

src/Service/Provider/ProviderInterface.php

@@ -6,8 +6,11 @@ namespace App\Service\Provider;
 
 interface ProviderInterface
 {
+    /** Fire the HTTP request(s) without blocking; returns an opaque handle for resolveFetch(). */
+    public function startFetch(): mixed;
+
     /** @return array<string, int>  date (Y-m-d) => contribution count */
-    public function fetch(): array;
+    public function resolveFetch(mixed $handle): array;
 
     public function isConfigured(): bool;