fix(ai): remove redundant g_strdup on api_key assignment

ai_get_provider_key() already returns a freshly g_strdup'd copy of
the API key. The outer g_strdup in ai_session_create() created an
unnecessary second copy, leaking the original pointer.

src/ai/ai_client.c

@@ -422,7 +422,7 @@ ai_session_create(const gchar* provider_name, const gchar* model)
     session->provider_name = g_strdup(provider_name);
     session->provider = ai_provider_ref(provider);
     session->model = g_strdup(model);
-    session->api_key = g_strdup(ai_get_provider_key(provider_name)); /* Own a copy of the key */
+    session->api_key = ai_get_provider_key(provider_name);
     session->history = NULL;
     session->ref_count = 1;