Once `pformat_caller_frame()` renders successfully, the default
`_mk_send_mte()` path still fails while formatting the valid IPC
msg spec and then constructs its error message as a one-element
tuple.
Pass `MsgCodec` to `pformat_msgspec()`, keep the assembled message
a string and exercise the complete path through a printable
`MsgTypeError` regression.
Prompt-IO: ai/prompt-io/opencode/20260820T150250Z_9afda1c6_prompt_io.md
(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))
Gud Boi