fix(openclaw): call() 5值→3值 — 修復全部 AI 分析降級根因

根因: _call_with_fallback() 回傳 5-tuple，但 call() 直接 return
導致呼叫端 (diagnostician/solver/critic agents) 3-var unpack
→ ValueError: too many values to unpack → 全部 降級 20%

修復: call() 明確解包再回傳 (response, provider, success)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/services/openclaw.py

@@ -816,8 +816,12 @@ class OpenClawService:
 
         Returns:
             (response, provider, success)
+
+        2026-04-16 ogt + Claude Sonnet 4.6: 修復 — _call_with_fallback 回傳 5 值，
+            call() 回傳 3 值，避免呼叫端 (diagnostician_agent:107) ValueError
         """
-        return await self._call_with_fallback(prompt)
+        response, provider, success, _tokens, _cost = await self._call_with_fallback(prompt)
+        return response, provider, success
 
     # =========================================================================
     # Fallback Chain