The #1630 fix introduced a blanket ``agent_failed_early`` transcript skip
to prevent context-overflow sessions from looping. That guard also
triggers for unrelated transient failures (429 rate limits, read
timeouts, connection resets, provider 5xx) which have nothing to do with
session size — and it silently drops the user's message, so the agent
has no memory of the last turn on retry.
Split the failure classification in ``GatewayRunner._run_agent``:
* Context-overflow (``compression_exhausted`` flag, explicit
context-length phrases, or generic 400 with a long history) → keep
the existing skip, preserving the #1630/#9893 fix.
* Anything else that failed → persist just the user message so the
conversation survives a retry.
Use specific multi-word phrases (``context length``, ``token limit``,
``prompt is too long``, etc.) to match ``run_agent.py``'s own
classifier; bare ``exceed`` false-positively flagged "rate limit
exceeded" as context overflow.
Covered by new tests in ``tests/gateway/test_7100_transient_failure_transcript.py``
and the existing #1630 suite still passes.
