manager/dispatcher: skip missing volumes when processing unpublish reports

[grey3228]

fixes #3208

- What I did

Fixed a nil pointer dereference in (*Dispatcher).processUpdates. When a node reports a volume as unpublished, and that volume is gone from the store by the time the report is processed, the leader manager panics.

UpdateVolumeStatus only queues the report until the next batch and doesn't check that the volume exists. processUpdates then logs volume unavailable but carries on and ranges over volume.PublishStatus.

One way to hit this: the volume is force-removed while a node is still unpublishing it. RemoveVolume with Force deletes the volume regardless of its PublishStatus. For example, a user removes a service, docker volume rm fails with "volume is still in use", and they retry with --force. The node then finishes unpublishing and reports it.

- How I did it

Return early from the batch callback if the volume doesn't exist. The same function already does this for nodes and tasks, and so do the scheduler (freeVolumes) and the CSI manager for missing volumes. There is nothing to update for a volume that no longer exists. Returning nil only skips that volume, so the rest of the batch is still processed.

- How to test it

go test ./manager/dispatcher/ -run TestVolumeUnpublishedDeletedVolume

The new test puts two volumes in PENDING_NODE_UNPUBLISH and queues unpublish reports for both. It then deletes one volume from the store and calls processUpdates. It checks that there is no panic and that the other volume still moves to PENDING_UNPUBLISH. Without the fix it fails with invalid memory address or nil pointer dereference.

The test doesn't start the dispatcher's Run loop on purpose. The loop processes updates every 100ms and could handle the reports before the volume is deleted, and then the test would pass without the fix.

- Description for the changelog

Fix a manager panic when a volume is removed before a node's unpublish report for it is processed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.75%. Comparing base (22c2c3d) to head (893fc4f).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3301      +/-   ##
==========================================
+ Coverage   14.71%   14.75%   +0.04%     
==========================================
  Files         198      198              
  Lines       93035    93034       -1     
==========================================
+ Hits        13690    13731      +41     
+ Misses      78011    77958      -53     
- Partials     1334     1345      +11     

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