feat(webui): enhance ConfigSchemaGenerator with field_docs and UI metadata

- Add AttrDocBase.get_class_field_docs() classmethod for class-level field docs extraction
- Merge json_schema_extra (x-widget, x-icon, step) into schema output
- Map Pydantic constraints (ge/le) to minValue/maxValue for frontend compatibility
- Add ge=0, le=1 constraints to ChatConfig.talk_value for validation

Completes Task 1 (including subtasks 1a, 1b, 1c, 1d) of webui-config-visualization-refactor plan.

src/config/official_configs.py

@@ -104,6 +104,8 @@ class ChatConfig(ConfigBase):
 
     talk_value: float = Field(
         default=1,
+        ge=0,
+        le=1,
         json_schema_extra={
             "x-widget": "slider",
             "x-icon": "message-circle",