fix(core): ensure references are cleared from idle scheduler - #70730

Caretaker note: presubmit failure is pre-existing.

Thank you for working on this. I reran the original issue reproduction against head d8645885bb5167df04732333774a8610f95550e2: the callback is removed, but both a one-time callback exception and a one-time _tick() exception still leave the expired idleId set. Remaining work and a fresh add() never receive a native callback (2 queued, 0 scheduled), in both idle-deadline and timeout-fallback modes. The no-error controls drain normally.

Full six-case results and exact reproduction adjustment: #70728 (comment)

Please also make clearing the in-flight handle exception-safe while preserving error propagation and re-entrancy protection, and cover the fresh-add-after-error case. The deadline-exhausted path deserves coverage because break inside finally can suppress a pending exception.