5123: Fix batch details r=dureuill a=irevoire
# Pull Request
## Related issue
Fixes https://github.com/meilisearch/meilisearch/issues/5079
Fixes https://github.com/meilisearch/meilisearch/issues/5112
## What does this PR do?
- Make the processing tasks actually processing in the stats of the batch instead of enqueued
- Stop counting one extra task for all non-prioritized batches in the stats
- Add a test
Co-authored-by: Tamo <tamo@meilisearch.com>