fix(seer): Propagate Explorer failure reasons (#122969)

Preserve `timeout` and `stalled` failure reasons when Sentry parses Seer
run state, allowing the Explorer chat API to expose the terminal cause
to the frontend.

Seer begins emitting these reasons in
[getsentry/seer#7911](https://github.com/getsentry/seer/pull/7911), but
Sentry's Pydantic boundary previously discarded the unknown field. The
field remains optional for compatibility with older Seer responses. This
unblocks the timeout recovery UI in #122934.