Issue #3586605: Replace opt-out [NO_SOURCES] marker with opt-in [SOURCES_USED]...

Paul Sheldrakerequested to merge
issue/ai_rag_search_chat-3586605:3586605-sources-used-opt-in-marker into 1.x

Issue #3586605: Replace opt-out [NO_SOURCES] marker with opt-in [SOURCES_USED] to prevent irrelevant sources on refusal responses

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3586605

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