fix: make Codex hooks transactional

elydora/plugins/_strict_json.py

@@ -0,0 +1,32 @@
+"""Strict JSON parsing with duplicate-key rejection."""
+
+from __future__ import annotations
+
+import json
+from typing import Any, Dict, List, Tuple
+
+
+JsonObject = Dict[str, Any]
+
+
+class _DuplicateKey(ValueError):
+    pass
+
+
+def _unique_object(pairs: List[Tuple[str, Any]]) -> JsonObject:
+    result: JsonObject = {}
+    for key, value in pairs:
+        if key in result:
+            raise _DuplicateKey(f'duplicate field "{key}"')
+        result[key] = value
+    return result
+
+
+def parse_json_object(raw: str, label: str) -> JsonObject:
+    try:
+        value = json.loads(raw, object_pairs_hook=_unique_object)
+    except (_DuplicateKey, json.JSONDecodeError) as error:
+        raise ValueError(f"Failed to parse {label}: {error}") from error
+    if not isinstance(value, dict):
+        raise ValueError(f"{label} must contain a JSON object")
+    return value