fix: duplicate GameServerSets not being cleaned up under a Fleet

[mnthe]

What type of PR is this?

/kind bug

What this PR does / Why we need it:

Fixes a bug in filterGameServerSetByActive() where duplicate GameServerSets sharing the same template as the Fleet are silently dropped — placed in neither the active nor the rest slice.

When multiple GameServerSets match the Fleet's template, the current code overwrites the active variable on each match (last-write-wins). Earlier matches are lost entirely: they don't go into rest, so applyDeploymentStrategy() and deleteEmptyGameServerSets() never touch them.

This causes two problems:

1. Orphaned GameServerSets that are never scaled down or deleted.
2. All duplicates converge to the Fleet's desired replica count,
   • because the Kubernetes informer store iterates a Go map with non-deterministic ordering — each syncFleet() can pick a different GSS as "active" and update its Spec.Replicas.

Screenshot — 3 duplicate GameServerSets under a single Fleet, all with the same Desired value:

The fix keeps the oldest GameServerSet (by CreationTimestamp) as active and places any duplicates into the rest slice, where the existing deployment strategy logic naturally scales them down and deletes them.

Which issue(s) this PR fixes:

Closes #4468

Special notes for your reviewer:

• Root cause is in filterGameServerSetByActive() (pkg/fleets/controller.go), which has had last-write-wins behavior since its initial implementation and has never been modified.
• Added TestControllerFilterGameServerSetByActiveDuplicates covering:
  • 3 duplicate-template GSS → oldest selected as active, others in rest
  • Non-deterministic list ordering → same deterministic result
  • Mix of matching and non-matching templates
• All existing tests pass (make test-go).
• Minimal change: 17 lines added to the function, 55 lines of new tests.

Does this PR introduce a user-facing change?

Fixed a bug where duplicate GameServerSets with the same template could
accumulate under a Fleet without being cleaned up, causing replicas to
multiply beyond the desired count.

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