Make document keyword search case-insensitive (LOWER(...) LIKE LOWER(?)) in the
list filter and SearchKnowledge / SearchKnowledgeInScopes paths so a query and
its differently-cased variant return the same results.
Classify not-found consistently as HTTP 404 (not 500) so clients read a
permanent resource.not_found instead of a retryable 5xx: map the document /
chunk not-found sentinels in the KB-access middleware (typed ErrChunkNotFound
via errors.Is), and return 404 for a missing message on the continue-stream and
message-delete endpoints.
nullkey
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