[dns-client] add destructor to ensure proper cleanup (#12026)

Adds a destructor to the `Dns::Client` class to ensure that `Stop()`
is called when a `Client` object is destroyed (freeing all allocated
queries).

This change prevents false memory leak reports from fuzzer tests when
an `ot::Instance` is destroyed during ongoing DNS queries (retries).

src/core/net/dns_client.hpp

@@ -674,6 +674,11 @@ public:
      */
     explicit Client(Instance &aInstance);
 
+    /**
+     * Destructor of `Client`
+     */
+    ~Client(void) { Stop(); }
+
     /**
      * Starts the DNS client.
      *