feat(guardrails): add pre-tool-call authorization middleware with pluggable providers (#1240)

Add GuardrailMiddleware that evaluates every tool call before execution.
Three provider options: built-in AllowlistProvider (zero deps), OAP passport
providers (open standard), or custom providers loaded by class path.

- GuardrailProvider protocol with GuardrailRequest/Decision dataclasses
- GuardrailMiddleware (AgentMiddleware, position 5 in chain)
- AllowlistProvider for simple deny/allow by tool name
- GuardrailsConfig (Pydantic singleton, loaded from config.yaml)
- 25 tests covering allow/deny, fail-closed/open, async, GraphBubbleUp
- Comprehensive docs at backend/docs/GUARDRAILS.md

Closes #1213

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/config/app_config.py

@@ -9,6 +9,7 @@ from pydantic import BaseModel, ConfigDict, Field
 
 from deerflow.config.checkpointer_config import CheckpointerConfig, load_checkpointer_config_from_dict
 from deerflow.config.extensions_config import ExtensionsConfig
+from deerflow.config.guardrails_config import load_guardrails_config_from_dict
 from deerflow.config.memory_config import load_memory_config_from_dict
 from deerflow.config.model_config import ModelConfig
 from deerflow.config.sandbox_config import SandboxConfig
@@ -107,6 +108,10 @@ class AppConfig(BaseModel):
         if "tool_search" in config_data:
             load_tool_search_config_from_dict(config_data["tool_search"])
 
+        # Load guardrails config if present
+        if "guardrails" in config_data:
+            load_guardrails_config_from_dict(config_data["guardrails"])
+
         # Load checkpointer config if present
         if "checkpointer" in config_data:
             load_checkpointer_config_from_dict(config_data["checkpointer"])