Encrypted reasoning blobs are sealed to the model that minted them, not
only to the endpoint. Switching models on the same custom Responses
endpoint therefore replayed blobs the new model cannot decrypt and the
turn failed with HTTP 400.
Stamp captured reasoning items with `_issuer_model` (the canonical wire
model) alongside `_issuer_kind`, and replay an item only when both the
issuer kind and the model match the current request. Endpoint-stamped
legacy items without model provenance are dropped once the current
model is known (fail closed); ordinary assistant text stays replayable.
The transport threads the effective wire model (request_overrides win)
into conversion and normalization; the auxiliary Codex adapter stamps
and filters against its own model rather than the main agent's. The
400 classifier also recognises the custom-endpoint wording
"encrypted content could not be decrypted or parsed" so recovery strips
the replay state instead of aborting.
Hand-grafted from #95849 (final head d9cf6bcc08) onto current main; the
middleware-model-rewrite half is intentionally left out.
Closes#95834
51ebdff570