NotFound(Source) error with S3 metastore and SQS ingestion after red/black deployment

Describe the bug
We're encountering a persistent issue where Quickwit fails with NotFound(Source) errors after several hours when using S3-based metastore and SQS ingestion, especially after a red/black deployment in Kubernetes.

For our red/black deployments, we have configured it in such a way that:

• When we started seeing the issue, we had 2 instances of the metastore running (1 per cluster). However, we have since tried a configuration that only has 1 metastore instance when the clusters overlap but this did not improve the situation.
• We only have one instance each of both the indexer and janitor services across both clusters

Unfortunately we're unable to concretely see what is causing this. After some time (sometimes a few hours), the indexer and control-plane start reporting:

failed to prune shards error=NotFound(Source { index_id: "logs", source_id: "sqs-logs-filesource" })
2025-05-28T22:30:15.186Z ERROR quickwit_actors::spawn_builder: actor-failure cause=source `logs/sqs-logs-filesource` not found exit_status=Failure(source `logs/sqs-logs-filesource` not found)
2025-05-28T22:30:15.186Z  INFO quickwit_actors::spawn_builder: actor-exit actor_id=SourceActor-winter-CheT exit_status=failure(cause=source `logs/sqs-logs-filesource` not found)
2025-05-28T22:30:15.186Z ERROR quickwit_actors::actor_context: exit activating-kill-switch actor=SourceActor-winter-CheT exit_status=Failure(source `logs/sqs-logs-filesource` not found)
2025-05-28T22:30:15.186Z  INFO quickwit_actors::spawn_builder: actor-exit actor_id=quickwit_indexing::actors::doc_processor::DocProcessor-empty-P75y exit_status=killed
2025-05-28T22:30:15.186Z  INFO quickwit_actors::spawn_builder: actor-exit actor_id=Indexer-ancient-wcY5 exit_status=killed

• All services (indexer, searcher, control-plane) complain that the source cannot be found.
• As a result, Quickwit does not ingest logs that are waiting in the queue and S3 buckets
• However, the metastore file in S3 correctly lists both the index and the source.
• Running quickwit CLI inside the pod can retrieve the source as expected.
• Restarting pods does not resolve the issue.
• Occasionally, re-uploading the S3 file and restarting services does fix the issue temporarily.

The issue appears to coincide with shard rebalancing:

indexer `quickwit-indexer-0` joined the cluster: rebalancing shards and rebuilding indexing plan

• Only occurs with SQS ingestion. Kafka ingestion and ingest API (for the same index) work without issue.
• Using a single S3 metastore, but red/black deployments may result in two metastores existing temporarily.
• No S3 locking is observed to be in play.
• Attempts to set polling_interval had no effect.
• The issue seems tied to shard/source distribution handled by get_shards_for_source_mut, which differs in implementation between file-backed and Postgres metastores.

We've noticed that:

• Manually downloading, modifying, and re-uploading the S3 metastore file — sometimes resolves issue temporarily.
• Using the quickwit CLI to confirm index/source visibility inside pod — confirms metastore contents are intact.

Steps to reproduce (if applicable)

Unfortunately we don't know how to reproduce this reliably. Sometimes it will fix itself after we redeploy the cluster and then break on its own within hours, or break after we do a red/black deployment.

Expected behavior
We expect the SQS configuration to continue working after deployments.

Configuration:
Please provide:

1. Output of quickwit --version: Quickwit 0.8.0 (x86_64-unknown-linux-gnu 2025-04-23T14:04:34Z 3a070c8)
2. The index_config.yaml:

doc_mapping:
      field_mappings:
      - fast: true
        fast_precision: nanoseconds
        input_formats:
        - iso8601
        - unix_timestamp
        name: timestamp
        output_format: unix_timestamp_secs
        type: datetime
      mode: dynamic
      timestamp_field: timestamp
    index_id: logs
    index_uri: s3://l0-<redacted>-use1-quickwit-indexes/indexes
    indexing_settings:
      commit_timeout_secs: 10
      merge_policy:
        max_merge_factor: 12
        max_merge_ops: 3
        merge_factor: 10
        type: limit_merge
      resources:
        max_merge_write_throughput: 80mb
    retention:
      period: 7 days
      schedule: '@daily'
    version: 0.8

For reference we also raised the issue in the Quickwit community discord: https://discord.com/channels/908281611840282624/1377414921490272487