refactor(config): eliminate global mutable state, wire DeerFlowContext into runtime

- Freeze all config models (AppConfig + 15 sub-configs) with frozen=True
- Purify from_file() — remove 9 load_*_from_dict() side-effect calls
- Replace mtime/reload/push/pop machinery with single ContextVar + init_app_config()
- Delete 10 sub-module globals and their getters/setters/loaders
- Migrate 50+ consumers from get_*_config() to get_app_config().xxx

- Expand DeerFlowContext: app_config + thread_id + agent_name (frozen dataclass)
- Wire into Gateway runtime (worker.py) and DeerFlowClient via context= parameter
- Remove sandbox_id from runtime.context — flows through ThreadState.sandbox only
- Middleware/tools access runtime.context directly via Runtime[DeerFlowContext] generic
- resolve_context() retained at server entry points for LangGraph Server fallback

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

@@ -1,11 +1,13 @@
 """Configuration for pre-tool-call authorization."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class GuardrailProviderConfig(BaseModel):
     """Configuration for a guardrail provider."""
 
+    model_config = ConfigDict(frozen=True)
+
     use: str = Field(description="Class path (e.g. 'deerflow.guardrails.builtin:AllowlistProvider')")
     config: dict = Field(default_factory=dict, description="Provider-specific settings passed as kwargs")
 
@@ -18,31 +20,9 @@ class GuardrailsConfig(BaseModel):
     agent's passport reference, and returns an allow/deny decision.
     """
 
+    model_config = ConfigDict(frozen=True)
+
     enabled: bool = Field(default=False, description="Enable guardrail middleware")
     fail_closed: bool = Field(default=True, description="Block tool calls if provider errors")
     passport: str | None = Field(default=None, description="OAP passport path or hosted agent ID")
     provider: GuardrailProviderConfig | None = Field(default=None, description="Guardrail provider configuration")
-
-
-_guardrails_config: GuardrailsConfig | None = None
-
-
-def get_guardrails_config() -> GuardrailsConfig:
-    """Get the guardrails config, returning defaults if not loaded."""
-    global _guardrails_config
-    if _guardrails_config is None:
-        _guardrails_config = GuardrailsConfig()
-    return _guardrails_config
-
-
-def load_guardrails_config_from_dict(data: dict) -> GuardrailsConfig:
-    """Load guardrails config from a dict (called during AppConfig loading)."""
-    global _guardrails_config
-    _guardrails_config = GuardrailsConfig.model_validate(data)
-    return _guardrails_config
-
-
-def reset_guardrails_config() -> None:
-    """Reset the cached config instance. Used in tests to prevent singleton leaks."""
-    global _guardrails_config
-    _guardrails_config = None