fix(ingester): Add WAL cleanup code

[paul1r]

What this PR does / why we need it:
This PR adds critical WAL cleanup functionality to the ingester to prevent disk space leaks and maintain a healthy checkpoint state.

1. Stale Temporary Checkpoint Cleanup

• Now scans and removes all .tmp checkpoint directories (not just the current one)
• Prevents accumulation of partial checkpoints from failed attempts
• Runs during ingester startup

2. Old Checkpoint Cleanup

• Identifies and removes checkpoints that reference WAL segments no longer present on disk
• Triggered when closing a checkpoint and at startup
• Cleans up old prior checkpoints that were not removed for whatever reason
• Always preserves at least the current checkpoint

Which issue(s) this PR fixes:
Without this cleanup logic, the WAL directory can accumulate:

• Failed temporary checkpoints that are never removed
• Orphaned checkpoint references pointing to deleted segments

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR