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.

nanobot/providers/factory.py

@@ -54,7 +54,7 @@ def _make_provider_core(
     if provider_name and not spec and p:
         if not p.api_base:
             raise ValueError(f"Provider '{provider_name}' requires api_base in config.")
-        spec = create_dynamic_spec(provider_name, thinking_style=p.thinking_style if p else "")
+        spec = create_dynamic_spec(provider_name, thinking_style=(p.thinking_style or "") if p else "")
     if spec and spec.is_transcription_only:
         raise ValueError(f"Provider '{provider_name}' only supports transcription.")
     backend = spec.backend if spec else "openai_compat"

nanobot/webui/settings_api.py

@@ -359,7 +359,7 @@ def _resolve_settings_provider(
     normalized = provider_name.replace("-", "_")
     for extra_name, provider_config in _dynamic_provider_items(config):
         if provider_name == extra_name or normalized == extra_name.replace("-", "_"):
-            return create_dynamic_spec(extra_name, thinking_style=provider_config.thinking_style), extra_name, provider_config
+            return create_dynamic_spec(extra_name, thinking_style=(provider_config.thinking_style or "")), extra_name, provider_config
     return None
 
 
@@ -739,7 +739,7 @@ def settings_payload(
         providers.append(
             _provider_settings_row(
                 provider_key,
-                create_dynamic_spec(provider_key, thinking_style=provider_config.thinking_style),
+                create_dynamic_spec(provider_key, thinking_style=(provider_config.thinking_style or "")),
                 provider_config,
             )
         )

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")