fix(gateway): remove unnecessary type assertion in buildAgentPrompt
fix(gateway): reject unsupported forced tool_choice modes
tool_choice=required and named function tool_choice are now rejected
with invalid_request_error until hard enforcement is implemented
at the agent runtime layer. Only auto and none remain supported.
docs: update Chat Completions tool_choice contract to match rejection behavior
Only auto and none are currently accepted; required and named
function tool_choice are rejected until hard enforcement exists.
0e6aca34db