Add dynamic variable support to Assistant model and related components

- Introduce dynamic variable definitions in AssistantConfig and Assistant models, allowing for flexible prompt customization.
- Implement validation for dynamic variable names and types in the schema.
- Update backend services and routes to handle dynamic variables in assistant configurations and runtime processing.
- Enhance frontend components to support dynamic variable definitions, including a new editor for managing variables.
- Add tests to ensure proper functionality and validation of dynamic variables in various scenarios.

backend/db/models.py

@@ -171,6 +171,7 @@ class Assistant(Base):
 
     # ---- 瘦类型专属字段(真列,稀疏:按 type 用其中几列) ----
     prompt: Mapped[str] = mapped_column(String(8192), default="")    # prompt / opencode
+    dynamic_variable_definitions: Mapped[dict] = mapped_column(JSON, default=dict)
     api_url: Mapped[str] = mapped_column(String(512), default="")    # dify / fastgpt / opencode
     api_key: Mapped[str] = mapped_column(String(512), default="")    # dify / fastgpt / opencode(打码/哨兵)
     app_id: Mapped[str] = mapped_column(String(128), default="")     # fastgpt