Fix cache key collision in Query Frontend with epoch-aware shard keys

[laurentketterle-hub]

Summary

During tenant shuffle-sharding rebalance, the Query Frontend intermittently merges cached query fragments generated by different shard assignments. Because the cache key generation did not account for the active shard configuration or transition state, fragments from the old shard assignment were mixed with fragments from the new shard assignment, resulting in partial/incomplete query results returned with HTTP 200 OK.

Fix

This PR implements epoch-aware cache keys in the Query Frontend:

1. ShardConfig and CacheKey types (pkg/queryfrontend/queryrange/types.go):

   • ShardConfig tracks a tenant's shard assignment with a version/epoch
   • ShardConfig.Hash() produces a deterministic, order-independent hash
   • CacheKey includes the shard epoch hash alongside tenant, query, time range, and step
   • ShardState tracks per-tenant configurations and transition status

2. QueryRangeCache middleware (pkg/queryfrontend/queryrange/cache.go):

   • BuildCacheKey() generates keys that incorporate the current shard epoch
   • Get() bypasses the cache during shard transitions to guarantee data completeness
   • InvalidateStaleEpoch() proactively removes cached entries from old epochs
   • InvalidateTenant() clears all cached entries for a tenant

3. Unit tests (pkg/queryfrontend/queryrange/cache_test.go):

   • Verifies cache keys change when shard configuration changes
   • Verifies cache is bypassed during transitions
   • Verifies stale epoch invalidation
   • Verifies hash determinism (order-independent)
   • Verifies different tenants produce different keys

How It Works

• When a query is cached, the cache key includes a hash of the tenant's current shard configuration and version
• When the shard assignment changes (e.g., scale-out from 3 to 5 shards), the hash changes
• New queries generate new cache keys, old fragments are never mixed with new ones
• During the transition window, the cache is bypassed entirely to ensure complete results
• After transition completes, InvalidateStaleEpoch() can clean up old epoch entries

Closes #1

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Signed-off-by: laurentketterle-hub noreply@users.noreply.github.com

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