fix: coalesce None thinking_style to empty string in provider creation

ProviderConfig.thinking_style defaults to None (Optional field), but
create_dynamic_spec expects a string. Coalesce None to "" at all call
sites (factory.py, settings_api.py) and fix the test assertion to
expect None from the config default.

tests/providers/test_custom_thinking_style.py

@@ -11,7 +11,7 @@ class TestCustomProviderThinkingStyle:
 
     def test_default_thinking_style_is_empty(self) -> None:
         cfg = ProviderConfig()
-        assert cfg.thinking_style == ""
+        assert cfg.thinking_style is None
 
     def test_create_dynamic_spec_default(self) -> None:
         spec = create_dynamic_spec("custom")