End chat 502'd with "escrow ref not on chain": endChat queried the original
Open escrow ref, but the supplier's Claim already spent it (Open->Claimed),
so queryUtxo returned null before the supplier was ever called. Now endChat
uses the supplier endpoint cached by the buyer-app at startChat, resolves
supplier identity from /capability for verification, and Accepts the Submitted
UTxO the supplier returns. Adds supplierBaseUrl to EndChatOptions; the
/v1/chat/end route passes it from chatRouteState (404 if the session is gone).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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