Fixes#93022. A short session (protection window >= transcript) hits the
"insufficient messages" / "no compressible window" branches twice and
permanently trips the anti-thrash breaker, even though nothing was
eligible to compress - compression was never attempted, so there is
nothing "ineffective" to score. The session then rides past the
threshold with no compaction possible (recovery probes only soften,
not fix, the misclassification).
Distinguish "nothing eligible right now" from "attempted and
underperformed":
- New transient _structural_no_op_backoff_until (in-memory, 300s)
armed by _record_structural_no_op() at the three structural no-op
sites: insufficient_messages, no_compressible_window,
empty_post_handoff_window. No strikes accumulate; auto-compaction
resumes on its own once the backoff lapses or the transcript outgrows
the protection window.
- The backoff gates should_compress via
_automatic_compression_blocked_locally and surfaces in
_compression_block_reason as "structural_backoff:<seconds>".
- #40803's frozen-CLI guarantee is preserved: a transcript that can
never shrink retries at most once per backoff window instead of
every turn.
- force=True (/compress) clears an active backoff before attempting;
record_completed_compaction() lifts it - both prove the transcript
is compressible/being worked.
- Genuine attempted-but-underperformed verdicts still strike the
durable ineffective counter unchanged.
Tests: new tests/agent/test_context_compressor_structural_backoff.py;
updated the two tests that asserted the old strike-on-noop behavior.
f778c0d941