fix: prefer compatible advertised model routes - #148

When a model advertises both Chat Completions and Responses, prefer Chat Completions. The exact saved OpenCode V2.0.18 session error was a response.output_item.added event whose function_call item lacked the required arguments string while DeepSeek V4.1 Flash #max was calling the shell tool. The earlier annotation-event description was incorrect.

This shared route policy applies by advertised endpoints, not by a DeepSeek model ID. In the current 84-model catalog, 66 entries advertise Chat and Responses, 10 advertise Messages only, and 8 advertise Chat only; no entry is Responses-only. Other models' Responses tool-call streams were not individually validated.

Validation after release:

• The installed local V2.0.18 binary loaded the working-tree plugin from an isolated file:// config and completed the same model/variant shell-tool flow (TOOL_CALL_SMOKE_OK).
• Official OpenCode V1.18.30 and V2.0.18 Docker images, also using the working-tree plugin, completed that tool call.
• CI passed on this PR; the exact old event remains captured in the provided session, not newly reproduced.