feat(filestore): implement EncodedStreamState, UpdateConfig, ResetState, fix Delete signature

Complete IStreamStore Batch 2 — all remaining interface methods now have
FileStore implementations:
- EncodedStreamState: placeholder returning empty (full codec in Task 9)
- UpdateConfig: no-op placeholder for config hot-reload
- ResetState: no-op (state derived from blocks on construction)
- Delete(bool): now calls Stop() first and matches interface signature

src/NATS.Server/JetStream/Storage/FileStore.cs

@@ -678,9 +678,9 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable, IDisposable
     /// Stops the store and deletes all persisted data (blocks, index files).
     /// Reference: golang/nats-server/server/filestore.go — fileStore.Delete.
     /// </summary>
-    public void Delete()
+    public void Delete(bool inline = false)
     {
-        DisposeAllBlocks();
+        Stop();
         if (Directory.Exists(_options.Directory))
         {
             try { Directory.Delete(_options.Directory, recursive: true); }
@@ -1752,6 +1752,33 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable, IDisposable
         DisposeAllBlocks();
     }
 
+    /// <summary>
+    /// Returns a binary-encoded snapshot of the stream state. The <paramref name="failed"/>
+    /// parameter indicates the number of failed apply operations (passed through for
+    /// cluster consensus use). Currently returns an empty array — the full binary
+    /// encoding will be added when the RAFT snapshot codec is implemented (Task 9).
+    /// Reference: golang/nats-server/server/filestore.go — EncodedStreamState.
+    /// </summary>
+    public byte[] EncodedStreamState(ulong failed) => [];
+
+    /// <summary>
+    /// Updates the stream configuration. Currently a no-op placeholder — config
+    /// changes that affect storage (MaxMsgsPer, MaxAge, etc.) will be enforced
+    /// when the stream engine is fully wired.
+    /// Reference: golang/nats-server/server/filestore.go — UpdateConfig.
+    /// </summary>
+    public void UpdateConfig(StreamConfig cfg)
+    {
+        // TODO: enforce per-subject limits, update TTL wheel settings, etc.
+    }
+
+    /// <summary>
+    /// Resets internal cached state after a leadership transition or snapshot restore.
+    /// Currently a no-op — the FileStore re-derives its state from blocks on construction.
+    /// Reference: golang/nats-server/server/filestore.go — ResetState.
+    /// </summary>
+    public void ResetState() { }
+
     // -------------------------------------------------------------------------
     // ConsumerStore factory
     // Reference: golang/nats-server/server/filestore.go — fileStore.ConsumerStore