feat: add response routing for service import request-reply patterns

src/NATS.Server/Imports/ResponseRouter.cs

@@ -0,0 +1,64 @@
+using System.Security.Cryptography;
+using NATS.Server.Auth;
+
+namespace NATS.Server.Imports;
+
+/// <summary>
+/// Handles response routing for service imports.
+/// Maps to Go's service reply prefix generation and response cleanup.
+/// Reference: golang/nats-server/server/accounts.go — addRespServiceImport, removeRespServiceImport
+/// </summary>
+public static class ResponseRouter
+{
+    private static readonly char[] Base62 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".ToCharArray();
+
+    /// <summary>
+    /// Generates a unique reply prefix for response routing.
+    /// Format: "_R_.{10 random base62 chars}."
+    /// </summary>
+    public static string GenerateReplyPrefix()
+    {
+        Span<byte> bytes = stackalloc byte[10];
+        RandomNumberGenerator.Fill(bytes);
+        var chars = new char[10];
+        for (int i = 0; i < 10; i++)
+            chars[i] = Base62[bytes[i] % 62];
+        return $"_R_.{new string(chars)}.";
+    }
+
+    /// <summary>
+    /// Creates a response service import that maps the generated reply prefix
+    /// back to the original reply subject on the requesting account.
+    /// </summary>
+    public static ServiceImport CreateResponseImport(
+        Account exporterAccount,
+        ServiceImport originalImport,
+        string originalReply)
+    {
+        var replyPrefix = GenerateReplyPrefix();
+
+        var responseSi = new ServiceImport
+        {
+            DestinationAccount = exporterAccount,
+            From = replyPrefix + ">",
+            To = originalReply,
+            IsResponse = true,
+            ResponseType = originalImport.ResponseType,
+            Export = originalImport.Export,
+            TimestampTicks = DateTime.UtcNow.Ticks,
+        };
+
+        exporterAccount.Exports.Responses[replyPrefix] = responseSi;
+        return responseSi;
+    }
+
+    /// <summary>
+    /// Removes a response import from the account's export map.
+    /// For Singleton responses, this is called after the first reply is delivered.
+    /// For Streamed/Chunked, it is called when the response stream ends.
+    /// </summary>
+    public static void CleanupResponse(Account account, string replyPrefix, ServiceImport responseSi)
+    {
+        account.Exports.Responses.Remove(replyPrefix);
+    }
+}