chatMessageToResponsesItems only mapped "system" explicitly and sent every
other unknown role through the user converter, so Chat Completions
developer messages reached the Responses upstream as user turns and lost
their instruction priority.
The Responses API accepts role "developer" in input messages; convert it
like a system message and keep the original role. Downstream converters
already understand it (ResponsesToAnthropicRequest folds developer into
the system prompt, the Codex transform forwards it unchanged).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>