buildSessionContext walked the entry path and unconditionally overwrote
models.default from every assistant message's reported model. Temporary
fallbacks (retry fallback, context promotion) and codex-side model
downgrades both produce assistant messages tagged with a different model
id, which clobbered the user's explicit /model pick on resume and made
the session silently revert to the older model.
Treat assistant-message inference as a legacy fallback that only fills
in models.default when no explicit `model_change` with role="default"
has been seen on the path.
Fixes#849
can1357
08403be71f