Codex Responses API strict mode forces every "optional" tool property into
`required`, so a model that intends to OMIT an optional enum property (no
declared `default`, e.g. Agent.isolation: enum["worktree","remote"]) must
still emit a concrete value. Neither of the two ops shipped in #6992
(drop-if-default, generalized drop-if-empty) can catch this: drop-if-default
needs a declared default (none exists); drop-if-empty needs an empty
string/array (the emitted value is non-empty).
Adds a paired request/response transform scoped strictly to
targetFormat === OPENAI_RESPONSES:
- Request side: injectOptionalEnumOmissionSentinel/-ForTools widen no-default
optional enum properties to accept `null` (OpenAI's own documented
nullable-union idiom for this exact strict-mode limitation).
- Response side: isDroppableNullEntry drops the key when the model emits
`null` for a non-required, schema-declared property, reusing the existing
#6992 toolSchemas plumbing (no new tracking structure needed).
