* fix(filer): remove cancellation guard from RollbackTransaction and clean up #8909
RollbackTransaction is a cleanup operation that must succeed even when
the context is cancelled — guarding it causes the exact orphaned state
that #8909 was trying to prevent.
Also:
- Use single-evaluation `if err := ctx.Err(); err != nil` pattern
instead of double-calling ctx.Err()
- Remove spurious blank lines before guards
- Add context.DeadlineExceeded test coverage
- Simplify tests from ~230 lines to ~130 lines
* fix(filer): call cancel() in expiredCtx and test rollback with expired context
- Call cancel() instead of suppressing it to avoid leaking timer resources
- Test RollbackTransaction with both cancelled and expired contexts
