fix(ecstore): don't exclude pool when has_space_for is indeterminate
When `has_space_for` returns an error (not enough online disks to
reliably determine space), the old code treated this the same as
"definitely no space" via `unwrap_or_default()`, zeroing out the
pool's available capacity. During pool decommission this creates a
false "Disk full" (500) for S3 PUT requests:
- Pool 0 (decommissioning) is correctly suspended → available = 0
- Pool 1 (active) has some disks whose disk_info call fails under
heavy migration I/O → has_space_for returns Err → available forced
to 0
- get_available_pool_idx sees total = 0 → returns None → DiskFull
Fix: distinguish Ok(false) (genuinely full) from Err (indeterminate).
On Err, log a warning and fall through to compute available space from
whatever disks did respond. The actual write will enforce its own
quorum; a premature zero at the pool-selection layer is a false
rejection.
Closes#5497
Zhengchao An