fix(agent-knowledge): send a valid _mget body when filtering generations
_active_generation_filter put _source at the top level alongside the
ids shorthand, but Elasticsearch only accepts docs or ids there and
answers with a parsing_exception 400. Since the call sits on the search
path, every agent knowledge search failed with a 500.
Switch to the docs form, which supports per-document _source, and add
a contract test whose ES fake rejects unknown top-level _mget keys the
way the real parser does — the previous fakes accepted any body, which is
why this slipped through CI.
Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com