fix(V2): map Deribit upstream 5xx / non-JSON to clean HTTPException 502

Cloudflare 5xx pages from Deribit testnet were leaking through the JSON
parser as JSONDecodeError → UNHANDLED_EXCEPTION. Wrap response parsing so
upstream errors surface as a retryable HTTP_502 envelope instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/exchanges/deribit/client.py

@@ -1,15 +1,35 @@
 from __future__ import annotations
 
 import contextlib
+import json
 import time
 from dataclasses import dataclass, field
 from typing import Any
 
+from fastapi import HTTPException
+
 from cerbero_mcp.common import indicators as ind
 from cerbero_mcp.common import microstructure as micro
 from cerbero_mcp.common import options as opt
 from cerbero_mcp.common.http import async_client
 
+
+def _parse_deribit_response(resp) -> dict:
+    """Map Deribit upstream errors to a clean HTTP 502 (retryable) instead of
+    leaking JSONDecodeError when the body is HTML (e.g. Cloudflare 5xx page)."""
+    if resp.status_code >= 500:
+        raise HTTPException(
+            status_code=502,
+            detail=f"Deribit upstream HTTP {resp.status_code}",
+        )
+    try:
+        return resp.json()
+    except json.JSONDecodeError as e:
+        raise HTTPException(
+            status_code=502,
+            detail=f"Deribit upstream returned non-JSON (status {resp.status_code})",
+        ) from e
+
 BASE_LIVE = "https://www.deribit.com/api/v2"
 BASE_TESTNET = "https://test.deribit.com/api/v2"
 
@@ -53,7 +73,7 @@ class DeribitClient:
         }
         async with async_client(timeout=15.0) as http:
             resp = await http.get(url, params=params)
-            data = resp.json()
+            data = _parse_deribit_response(resp)
         if "result" not in data:
             error = data.get("error", {})
             msg = error.get("message", str(data)) if isinstance(error, dict) else str(error)
@@ -87,7 +107,7 @@ class DeribitClient:
 
         async with async_client(timeout=15.0) as http:
             resp = await http.get(url, params=request_params, headers=headers)
-            data = resp.json()
+            data = _parse_deribit_response(resp)
 
             if "result" not in data:
                 error = data.get("error", {})
@@ -99,7 +119,7 @@ class DeribitClient:
                     await self._authenticate()
                     headers["Authorization"] = f"Bearer {self._token}"
                     resp = await http.get(url, params=request_params, headers=headers)
-                    data = resp.json()
+                    data = _parse_deribit_response(resp)
                     if "result" in data:
                         return data  # type: ignore[no-any-return]
                 return {"result": None, "error": error_msg}