Fix compactor concurrency limit not enforced across compaction levels

[adamalexandru4]

What this PR does

BucketCompactor.Compact() has a for loop that repeatedly calls grouper.Groups() after each successful compaction (shouldRerun=true). The grouper returns up to compactionConcurrency groups per call, but since the loop calls it multiple times, the effective limit is unbounded.

With concurrency=1 and blocks [6×2h, 2×12h]:

• Iteration 1: Groups() → 1 group (6×2h → 12h), compacts, shouldRerun=true
• Iteration 2: Groups() → 1 group (3×12h → 24h), compacts, shouldRerun=true
• Iteration 3: Groups() → 0 groups, loop breaks

Result: 2 compactions in one pass despite concurrency=1. Each downloads blocks to local disk, causing unexpected disk usage spikes.

Fix

Track cumulative groups returned across Groups() calls within one Compact() invocation. Once the total reaches compactionConcurrency, return empty:

func (g *ShuffleShardingGrouper) Groups(blocks map[ulid.ULID]*metadata.Meta) ([]*compact.Group, error) {
    remainingConcurrency := g.compactionConcurrency - g.totalGroupsPlanned
    if remainingConcurrency <= 0 {
        return nil, nil
    }
    // ... existing logic, using remainingConcurrency as the limit ...
    g.totalGroupsPlanned += len(outGroups)
    return outGroups, nil
}

This is safe because the grouper is created fresh in each compactUser() call, so totalGroupsPlanned resets every pass.

The fix currently covers:

• ShuffleShardingGrouper (sharding_strategy: shuffle-sharding)
• PartitionCompactionGrouper (sharding_strategy: shuffle-sharding + compaction_strategy: partitioning)

Gap: The DefaultBlocksGrouperFactory path (used when sharding_strategy: default or sharding is disabled) delegates to the Thanos DefaultGrouper, which is not patched. This affects single-tenant / non-sharded setups. Plan is to wrap it in a concurrency-limiting adapter.

Tests

• Unit tests for both ShuffleShardingGrouper and PartitionCompactionGrouper: call Groups() twice with concurrency=1, assert second call returns 0 groups
• Integration test reproducing the exact bug scenario (6×2h + 2×12h blocks)

Which issue(s) this PR fixes:
Fixes [#7298]

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]