Live proxy suites for the sail provider added in #42840: chat with
service_tier flex, balanced and absent bills each window's cost-map
rates read at run time and echoes no service_tier; /v1/responses with
extra_body.metadata.completion_window balanced bills the _balanced
keys; a flex-only sail model refuses the asap and balanced windows
with sail's own 400 and leaves only a zero-spend failure row; openai
keeps its own service_tier and bills the tier it reports back.
MUTATIONS.md records the five production mutations, each killed, and
the final green run. It is review evidence and not meant to merge.
