Fix data race when migrating slots

[greatsharp]

1. update the migrate status of each shard separately,
2. migrate slot only should clear the migration info.
3. invoke MigrateSlot API serially to resolve issue 282

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.82759% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.08%. Comparing base (6c56470) to head (77bcab6).
⚠️ Report is 77 commits behind head on unstable.

Files with missing lines	Patch %	Lines
controller/cluster.go	0.00%	9 Missing ⚠️
server/api/cluster.go	76.47%	3 Missing and 1 partial ⚠️
store/cluster.go	0.00%	2 Missing ⚠️
server/route.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #324      +/-   ##
============================================
+ Coverage     43.38%   47.08%   +3.69%     
============================================
  Files            37       45       +8     
  Lines          2971     4420    +1449     
============================================
+ Hits           1289     2081     +792     
- Misses         1544     2130     +586     
- Partials        138      209      +71     

Flag	Coverage Δ
unittests	47.08% <44.82%> (+3.69%)	⬆️

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[Copilot]

Pull Request Overview

This PR aims to resolve migration inconsistencies by resetting shard migration state per-shard, ensuring slot-only migrations clear previous migration info, and serializing the MigrateSlot API to avoid concurrent conflicts.

• Clear migration state for source shard on slot-only migrations
• Serialize MigrateSlot API calls with a handler-level mutex and manual cluster lookup
• Adjust routing middleware and change loop error handling to continue on per-shard failures

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
store/cluster.go	Clear migration state before moving slots when slotOnly is true
server/route.go	Removed RequiredCluster middleware from the migrate endpoint
server/api/cluster.go	Added sync.Mutex, switched to manual cluster retrieval, and serialized MigrateSlot calls
controller/cluster.go	Replaced early return with continue in shard status update loop

Comments suppressed due to low confidence (4)

server/route.go:72

• The migrate route no longer includes RequiredNamespace and RequiredCluster middleware, which may bypass essential validation and context injection. Consider re-adding the appropriate middleware to enforce namespace and cluster context.

			clusters.POST("/:cluster/migrate", handler.Cluster.MigrateSlot)

server/api/cluster.go:50

• [nitpick] The field name mu is generic; consider renaming it to something more descriptive like migrateMu or clusterMu to clarify its purpose.

	mu sync.Mutex

store/cluster.go:210

• [nitpick] It may be helpful to add a comment explaining why ClearMigrateState is only called for the source shard when slotOnly is true, to improve maintainability and clarity of intent.

		cluster.Shards[sourceShardIdx].ClearMigrateState()

server/api/cluster.go:123

• Using a single mutex on the handler serializes all MigrateSlot calls across all clusters, potentially creating a bottleneck. Consider using a more granular lock mechanism (e.g., per-cluster) to allow concurrent migrations on different clusters.

	handler.mu.Lock()