fix(tools): keep an empty required: [] in sanitized tool schemas

`_sanitize_node` deleted the `required` key whenever the pruned list came
out empty. Four built-in tools (skills_list, todo, delegate_task,
session_search) declare `required: []`, so they left the sanitizer with no
key at all. Strict OpenAI-compatible proxies read the missing key as
`null` and 400 the whole request ("null is not of type array"), which is
non-retryable and kills the session on its first call.

An empty array is valid for every backend; the pruning was added (34c3e67)
to drop names that are not in `properties`, not to delete the key. Keep the
key with the filtered list, even when that list is empty.

Fixes #111684
Fixes #59386
Co-authored-by: Cr4ckMe <jiqing.liu@whu.edu.cn>

tools/schema_sanitizer.py

@@ -315,11 +315,9 @@ def _sanitize_node(node: Any, path: str) -> Any:
         if not isinstance(out.get("properties"), dict):
             out["properties"] = {}
         if isinstance(out.get("required"), list):
-            valid = [r for r in out["required"] if isinstance(r, str) and r in out["properties"]]
-            if valid:
-                out["required"] = valid
-            else:
-                del out["required"]
+            # Keep the key even when nothing survives: ``required: []`` is valid everywhere,
+            # while a missing key reads as ``null`` on strict OpenAI-compatible proxies.
+            out["required"] = [r for r in out["required"] if isinstance(r, str) and r in out["properties"]]
     return out