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