With ide=windsurf, GetChatMessage rejects every GPT-5.6 variant with
'This model is only in Devin Local.' Switching the client identity to
devin-desktop makes the server serve them; response-header wire echo
confirms the exact model (gpt-5-6-sol-high -> 'GPT-5.6 Sol High Thinking').
Unknown UIDs still hard-error, so there is no silent fallback.
