The preflight-compression gate only ran the (expensive) token estimate when
the message COUNT exceeded protect_first_n + protect_last_n + 1. A session
with a handful of very large messages never tripped the count condition, so
compression was never attempted and the turn eventually hit a hard
context-overflow error.
Add _should_run_preflight_estimate() with OR semantics: run the estimate when
either the message count exceeds the protected ranges (the historical gate)
OR a cheap char-based estimate already crosses the configured threshold. The
downstream estimate_request_tokens_rough() stays authoritative — this is only
a hint that decides whether to pay for the full estimate.
Salvaged from #27435 by @texhy (authorship preserved). Re-applied on current
main: the preflight gate moved from conversation_loop.py to turn_context.py
since the PR was opened, so the helper + gate are placed there; the test
imports the real MINIMUM_CONTEXT_LENGTH instead of a hardcoded literal.
Closes#27405.
