stripEmptyOptionalToolArgs was allowlist-only (Read/Subagent) and only
stripped empty-string/empty-array values, so Responses API strict mode
(every property forced into `required`) could forward a forced non-empty
value (e.g. Agent.isolation) or a schema-declared default value verbatim
to the client. Add schema-aware drop-if-default and generalized
drop-if-empty (any tool, gated on schema.required), and thread each
tool's JSON Schema from the request's tools[] into the two streaming
call sites (response.output_item.done handling).
Closes#6951