fix: check freeEcSlot before evacuating EC shards to prevent data loss (#7621)

* fix: check freeEcSlot before evacuating EC shards to prevent data loss

Related to #7619

The moveAwayOneEcVolume function was missing the freeEcSlot check that
exists in other EC shard placement functions. This could cause EC shards
to be moved to volume servers that have no capacity, resulting in:
1. 0-byte shard files when disk is full
2. Data loss when source shards are deleted after 'successful' copy

Changes:
- Add freeEcSlot check before attempting to move EC shards
- Sort destinations by both shard count and free slots
- Refresh topology during evacuation to get updated slot counts
- Log when nodes are skipped due to no free slots
- Update freeEcSlot count after successful moves

* fix: clarify comment wording per CodeRabbit review

The comment stated 'after each move' but the code executes before
calling moveAwayOneEcVolume. Updated to 'before moving each EC volume'
for accuracy.

* fix: collect topology once and track capacity changes locally

Remove the topology refresh within the loop as it gives a false sense
of correctness - the refreshed topology could still be stale (minutes old).

Instead, we:
1. Collect topology once at the start
2. Track capacity changes ourselves via freeEcSlot decrement after each move

This is more accurate because we know exactly what moves we've made,
rather than relying on potentially stale topology refreshes.

* fix: ensure partial EC volume moves are reported as failures

Set hasMoved=false when a shard fails to move, even if previous shards
succeeded. This prevents the caller from incorrectly assuming the entire
volume was evacuated, which could lead to data loss if the source server
is decommissioned based on this incorrect status.

* fix: also reset hasMoved on moveMountedShardToEcNode error

Same issue as the previous fix: if moveMountedShardToEcNode fails
after some shards succeeded, hasMoved would incorrectly be true.
Ensure partial moves are always reported as failures.

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