feat: add reverse response mapping for cross-account request-reply (Gap 9.9)

src/NATS.Server/Auth/Account.cs

@@ -737,6 +737,46 @@ public sealed class Account : IDisposable
         return new AccountClaimUpdateResult(Changed: false, ChangedFields: [], UpdateCount: Volatile.Read(ref _claimUpdateCount));
     }
 
+    // Reverse response map for cross-account request-reply routing.
+    // When a service import rewrites the reply subject, a reverse mapping is stored so that
+    // when the response arrives it can be forwarded back to the original requester's account.
+    // Go reference: server/accounts.go — addRespMapEntry / checkForReverseEntries.
+    private readonly ConcurrentDictionary<string, ReverseResponseMapEntry> _reverseResponseMap =
+        new(StringComparer.Ordinal);
+
+    /// <summary>
+    /// Adds (or overwrites) a reverse response mapping for <paramref name="replySubject"/>.
+    /// Records which origin account and original reply subject to route the response back to.
+    /// Go reference: accounts.go addRespMapEntry.
+    /// </summary>
+    public void AddReverseRespMapEntry(string replySubject, string originAccount, string originalReply) =>
+        _reverseResponseMap[replySubject] = new ReverseResponseMapEntry(
+            replySubject, originAccount, originalReply, DateTime.UtcNow);
+
+    /// <summary>
+    /// Looks up the reverse response map entry for <paramref name="replySubject"/>.
+    /// Returns <see langword="null"/> when no mapping exists.
+    /// Go reference: accounts.go checkForReverseEntries.
+    /// </summary>
+    public ReverseResponseMapEntry? CheckForReverseEntries(string replySubject) =>
+        _reverseResponseMap.TryGetValue(replySubject, out var entry) ? entry : null;
+
+    /// <summary>
+    /// Removes the reverse response mapping for <paramref name="replySubject"/>.
+    /// Returns <see langword="true"/> if the entry was found and removed.
+    /// </summary>
+    public bool RemoveReverseRespMapEntry(string replySubject) =>
+        _reverseResponseMap.TryRemove(replySubject, out _);
+
+    /// <summary>The number of active reverse response map entries.</summary>
+    public int ReverseResponseMapCount => _reverseResponseMap.Count;
+
+    /// <summary>Removes all reverse response map entries.</summary>
+    public void ClearReverseResponseMap() => _reverseResponseMap.Clear();
+
+    /// <summary>Returns a snapshot of all reply subjects currently in the reverse response map.</summary>
+    public IReadOnlyList<string> GetReverseResponseMapKeys() => [.. _reverseResponseMap.Keys];
+
     public void Dispose() => SubList.Dispose();
 }
 
@@ -861,3 +901,19 @@ public sealed record AccountClaimUpdateResult(
     bool Changed,
     IReadOnlyList<string> ChangedFields,
     int UpdateCount);
+
+/// <summary>
+/// A single entry in an account's reverse response map.
+/// Maps a rewritten reply subject back to the original requester's account and subject
+/// so that service responses can be routed across account boundaries.
+/// Go reference: server/accounts.go — respMapEntry struct used by addRespMapEntry / checkForReverseEntries.
+/// </summary>
+/// <param name="ReplySubject">The rewritten reply subject used by the service provider.</param>
+/// <param name="OriginAccount">The name of the account that originated the request.</param>
+/// <param name="OriginalReply">The reply subject the originating client is listening on.</param>
+/// <param name="CreatedAt">The UTC time at which this mapping was recorded.</param>
+public sealed record ReverseResponseMapEntry(
+    string ReplySubject,
+    string OriginAccount,
+    string OriginalReply,
+    DateTime CreatedAt);