feat(cluster): add stream/consumer assignment processing with validation

Add 5 validated Process* methods to JetStreamMetaGroup for stream and
consumer assignment processing: ProcessStreamAssignment, ProcessUpdateStreamAssignment,
ProcessStreamRemoval, ProcessConsumerAssignment, and ProcessConsumerRemoval.
Each returns a bool indicating success, with validation guards matching
Go reference jetstream_cluster.go:4541-5925. Includes 12 new unit tests.

src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs

@@ -264,6 +264,101 @@ public sealed class JetStreamMetaGroup
         return Task.CompletedTask;
     }
 
+    // ---------------------------------------------------------------
+    // Validated assignment processing
+    // Go reference: jetstream_cluster.go:4541-5925
+    // ---------------------------------------------------------------
+
+    /// <summary>
+    /// Validates and processes a stream assignment.
+    /// Returns false if the assignment is invalid (empty name, null group).
+    /// Idempotent: duplicate assignments for the same stream name are accepted.
+    /// Go reference: jetstream_cluster.go:4541 processStreamAssignment.
+    /// </summary>
+    public bool ProcessStreamAssignment(StreamAssignment sa)
+    {
+        if (string.IsNullOrEmpty(sa.StreamName) || sa.Group == null)
+            return false;
+
+        AddStreamAssignment(sa);
+        return true;
+    }
+
+    /// <summary>
+    /// Updates an existing stream assignment's configuration.
+    /// Returns false if the stream does not exist.
+    /// Go reference: jetstream_cluster.go processUpdateStreamAssignment.
+    /// </summary>
+    public bool ProcessUpdateStreamAssignment(StreamAssignment sa)
+    {
+        if (!_assignments.TryGetValue(sa.StreamName, out var existing))
+            return false;
+
+        // Update the config while preserving consumers and other state
+        var updated = new StreamAssignment
+        {
+            StreamName = sa.StreamName,
+            Group = sa.Group,
+            ConfigJson = sa.ConfigJson,
+            Created = existing.Created,
+            SyncSubject = existing.SyncSubject,
+        };
+        // Copy consumers from old to new
+        foreach (var (name, ca) in existing.Consumers)
+            updated.Consumers[name] = ca;
+
+        _assignments[sa.StreamName] = updated;
+        return true;
+    }
+
+    /// <summary>
+    /// Removes a stream and all its consumers.
+    /// Returns false if stream didn't exist. Returns true if removed.
+    /// Go reference: jetstream_cluster.go processStreamRemoval.
+    /// </summary>
+    public bool ProcessStreamRemoval(string streamName)
+    {
+        if (!_assignments.ContainsKey(streamName))
+            return false;
+
+        RemoveStreamAssignment(streamName);
+        return true;
+    }
+
+    /// <summary>
+    /// Validates and processes a consumer assignment.
+    /// Returns false if the parent stream does not exist or consumer name is empty.
+    /// Go reference: jetstream_cluster.go:5300 processConsumerAssignment.
+    /// </summary>
+    public bool ProcessConsumerAssignment(ConsumerAssignment ca)
+    {
+        if (string.IsNullOrEmpty(ca.ConsumerName) || string.IsNullOrEmpty(ca.StreamName))
+            return false;
+
+        if (!_assignments.ContainsKey(ca.StreamName))
+            return false;
+
+        AddConsumerAssignment(ca.StreamName, ca);
+        return true;
+    }
+
+    /// <summary>
+    /// Removes a consumer assignment.
+    /// Returns false if stream or consumer doesn't exist.
+    /// Go reference: jetstream_cluster.go processConsumerRemoval.
+    /// </summary>
+    public bool ProcessConsumerRemoval(string streamName, string consumerName)
+    {
+        if (!_assignments.TryGetValue(streamName, out var sa))
+            return false;
+
+        if (!sa.Consumers.ContainsKey(consumerName))
+            return false;
+
+        RemoveConsumerAssignment(streamName, consumerName);
+        return true;
+    }
+
     // ---------------------------------------------------------------
     // Monitor-facing mutation methods
     // Called by JetStreamClusterMonitor when processing committed RAFT entries.