SessionPrune.resetThresholds leaves stale checkpoint writer failure counters

[bcoderg-sys]

Problem

SessionPrune keeps several pieces of per-session checkpoint bookkeeping: crossed, maxCrossed, and writerFailures.

resetThresholds(sessionID) currently clears crossed and maxCrossed, but it leaves writerFailures untouched. Since writerFailures is keyed by the same sessionID, the same session can enter a fresh checkpoint threshold cycle while still carrying the previous consecutive writer failure count.

If the previous count is already close to max_writer_failures, a later checkpoint writer failure in that same session can reach the retry cap earlier than expected. It also leaves a small stale in-memory entry until a successful writer clears it or the process restarts.

Expected behavior

When resetThresholds(sessionID) starts a fresh checkpoint threshold cycle for a session, all related per-session checkpoint threshold state should be reset together.

Suggested fix

Clear the writer failure counter together with the other reset state:

writerFailures.delete(sessionID)

Suggested test coverage

Add a targeted SessionPrune.fireCheckpoints test that:

1. records two checkpoint writer failures for a session,
2. calls resetThresholds(sessionID),
3. verifies the next checkpoint threshold cycle starts with a fresh failure count instead of inheriting the previous value.

This keeps the fix scoped to the same session and avoids changing retry behavior across different sessions.