Return false when an unless_exist write fails

[tadhgboyle]

Problem

write(..., unless_exist: true) sets written = true inside the lock_and_write block, before the entry is persisted. If persistence then raises a handled database error, Failsafe returns nil, but write_entry ignores that result and still returns true.

This also causes the local cache to retain a value that was never persisted. Callers using the return value to acquire a lock can proceed without owning it.

Fix

Derive written from the result of entry_lock_and_write instead of mutating it inside the block. A completed write returns its serialized payload and becomes true; an existing unexpired entry or a handled failure returns nil and becomes false.

Tests

Added regressions for:

• A deadlock during persistence returning false rather than true.
• A statement timeout during persistence not populating the local cache.