Tighten Matrix agent registry validation

adapter/matrix/agent_registry.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
-from dataclasses import dataclass
 from collections.abc import Mapping
+from dataclasses import dataclass
 from pathlib import Path
 
 import yaml
@@ -29,8 +29,20 @@ class AgentRegistry:
             raise AgentRegistryError(f"unknown agent id: {agent_id}") from exc
 
 
+def _load_registry_data(path: str | Path) -> dict[str, object]:
+    try:
+        raw = yaml.safe_load(Path(path).read_text(encoding="utf-8"))
+    except yaml.YAMLError as exc:
+        raise AgentRegistryError("invalid agent registry YAML") from exc
+    if raw is None:
+        return {}
+    if not isinstance(raw, Mapping):
+        raise AgentRegistryError("agent registry must be a mapping with an agents list")
+    return dict(raw)
+
+
 def load_agent_registry(path: str | Path) -> AgentRegistry:
-    raw = yaml.safe_load(Path(path).read_text(encoding="utf-8")) or {}
+    raw = _load_registry_data(path)
     entries = raw.get("agents")
     if not isinstance(entries, list) or not entries:
         raise AgentRegistryError("agents registry must contain a non-empty agents list")