tests(ai_client): remove remaining ai_provider_unref() before ai_client_shutdown()

Fix three more tests that called ai_provider_unref() on providers owned
by the hash table, causing use-after-free when ai_client_shutdown()
destroyed the hash table.

Tests fixed:
- test_ai_parse_models_null_handling
- test_ai_parse_models_escaped_quotes
- test_ai_parse_models_with_whitespace

tests/unittests/test_ai_client.c

@@ -834,7 +834,6 @@ test_ai_parse_models_null_handling(void** state)
     ai_parse_models_from_json(provider, NULL);
     assert_null(provider->models);
 
-    ai_provider_unref(provider);
     ai_client_shutdown();
 }
 
@@ -858,7 +857,6 @@ test_ai_parse_models_escaped_quotes(void** state)
     GList* models = provider->models;
     assert_non_null(models);
 
-    ai_provider_unref(provider);
     ai_client_shutdown();
 }
 
@@ -882,7 +880,6 @@ test_ai_parse_models_with_whitespace(void** state)
     GList* models = provider->models;
     assert_string_equal("model-with-spaces", models->data);
 
-    ai_provider_unref(provider);
     ai_client_shutdown();
 }