Relocates the #20424 wiring: the preflight region moved out of
run_agent.py into agent/turn_context.py (and through the
compression.max_attempts unification, #69315), so the contributor's
elif branch is reapplied at its current home as the else arm of the
threshold dispatch chain.
Integration contracts:
- Byte-identical default: the built-in ContextCompressor inherits
ContextEngine.should_compress_preflight() -> False, so the default
path performs no compression and touches no turn bookkeeping
(pinned by test_builtin_compressor_default_sub_threshold_path_unchanged).
- Attempt-cap: the engine gets exactly ONE compress() pass per turn,
mutually exclusive with the cap-bounded threshold multi-pass loop,
so turn-start passes stay within the resolved
compression.max_attempts budget in every case.
- No-op blocking (#64382 / 377244f7c): an engine pass that no-ops
(_compress_context returns the input list object) neither sets nor
clears preflight_compression_blocked and does not re-baseline the
flush history — a sub-threshold maintenance no-op proves nothing
about over-threshold compressibility.
- Engine exceptions are swallowed at debug level; cooldown/defer/
codex-native gates run before the hook is ever consulted.
Salvaged from #20424 by @Beandon13. Fixes#20316.
