Lowering the session trigger must not replace the window-relative lean
selection budget with threshold times target_ratio. Invalidate the lean
cache through the existing property while preserving explicit legacy and
external-engine fallback behavior.
Narrow adaptation of the aux-sync diagnosis and invariants in #93576,
without adding a required recalibration method to context engines.
Related: #95681, #93576
Co-authored-by: Turgut Kural <58116817+TurgutKural@users.noreply.github.com>