fix: accept blank tool call arguments

Treat blank OpenAI-compatible tool call arguments as an empty dict so parameterless tools such as finish can execute with providers that return an empty string. Also trim model identifiers during config normalization to avoid leading whitespace leaking into requests and snapshots.

pytests/config_test/test_model_info_normalization.py

@@ -0,0 +1,11 @@
+from src.config.model_configs import ModelInfo
+
+
+def test_model_identifier_strips_surrounding_whitespace() -> None:
+    model_info = ModelInfo(
+        api_provider="test-provider",
+        model_identifier=" glm-5.1 ",
+        name="test-model",
+    )
+
+    assert model_info.model_identifier == "glm-5.1"