fix(ai): 修復 Pydantic validation error + tuple unpacking

1. kubectl_command 允許 None (LLM 可能返回 null)
2. 加入 field_validator 將 null 轉換為空字串
3. generate_incident_proposal 完整解包 6 值 (含 ai_tokens/ai_cost)

2026-03-29 ogt: Gemini API validation 修復

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/models/ai.py

@@ -94,11 +94,18 @@ class OpenClawDecision(BaseModel):
         default="default",
         description="Kubernetes namespace",
     )
-    kubectl_command: str = Field(
+    # 2026-03-29 ogt: 允許 None，LLM 可能返回 null
+    kubectl_command: str | None = Field(
         default="",
         description="具體的 kubectl 指令",
     )
 
+    @field_validator("kubectl_command", mode="before")
+    @classmethod
+    def normalize_kubectl_command(cls, v):
+        """將 null 轉換為空字串"""
+        return v if v is not None else ""
+
     # === 風險評估欄位 ===
     risk_level: AIRiskLevel = Field(
         ...,