fix(gateway): a provider refusing the task's earlier reasoning is said in words (#147)

* fix(gateway): a provider refusing the task's earlier reasoning is said in words

OpenAI's encrypted reasoning items are opened only by the account that produced
them; a route served from more than one upstream account hands the next turn
items its upstream cannot open, and the person read the provider's raw JSON.
Measured on TokenRouter's gpt-5.4 route: a gpt-5.4 thread switched into
gpt-6-astra (the codex column, 2026-09-10) while six other ids made the same
switch and passed, and a same-model cold restore on hosted (2026-09-08). The
turn now fails in words that say what to do, with the previous model named when
there is one; the provider's JSON is never the message. Ported from hosted 2f18cfdb.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ej7eMwxnexLFqkiFzHq9Fk

* chore: start checks

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ej7eMwxnexLFqkiFzHq9Fk

---------

Co-authored-by: richard-epsilla <richard@epsilla.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>