refactor: Simplify managedQueue to a decorator

Removes the complex, stateful signaling mechanism from the
`managedQueue`.
Its sole responsibility is now to decorate a `SafeQueue` with atomic,
strictly consistent statistics tracking.

This change is the foundational first step in moving the registry from a
complex, event-driven lifecycle model to a simpler, lease-based one.
The now-unused event and lifecycle types are also removed.

Author: LukeAVanDrie

pkg/epp/flowcontrol/contracts/errors.go

@@ -33,4 +33,8 @@ var (
 
 	// ErrInvalidShardCount indicates that an invalid shard count was provided (e.g., zero or negative).
 	ErrInvalidShardCount = errors.New("invalid shard count")
+
+	// ErrShardDraining indicates that an operation could not be completed because the target shard is in the process of
+	// being gracefully drained. The caller should retry the operation on a different, Active shard.
+	ErrShardDraining = errors.New("shard is draining")
 )

pkg/epp/flowcontrol/contracts/registry.go

@@ -163,14 +163,16 @@ type RegistryShard interface {
 	Stats() ShardStats
 }
 
-// ManagedQueue defines the interface for a flow's queue instance on a specific shard.
-// It acts as a stateful decorator around an underlying `framework.SafeQueue`.
+// ManagedQueue defines the interface for a flow's queue on a specific shard.
+// It acts as a stateful decorator around an underlying `framework.SafeQueue`, augmenting it with statistics tracking.
 //
 // # Conformance
 //
-//   - All methods MUST be goroutine-safe.
-//   - All mutating methods (`Add()`, `Remove()`, etc.) MUST ensure that the underlying queue state and the statistics
-//     (`Len`, `ByteSize`) are updated atomically relative to each other.
+//   - Implementations MUST be goroutine-safe.
+//   - All mutating methods MUST ensure that the underlying queue state and the public statistics (`Len`, `ByteSize`)
+//     are updated as a single atomic transaction.
+//   - The `Add` method MUST return an error wrapping `ErrShardDraining` if the queue instance belongs to a parent shard
+//     that is no longer Active.
 type ManagedQueue interface {
 	framework.SafeQueue