feat: add stream import cycle detection (Gap 9.3)

Add StreamImportFormsCycle DFS method to Account plus GetStreamImportSources
and HasStreamImportFrom helpers. Add GetStreamImportSourceAccounts to ImportMap.
10 tests cover direct, indirect, self, diamond, and empty import scenarios.

src/NATS.Server/Imports/ImportMap.cs

@@ -1,3 +1,5 @@
+using NATS.Server.Auth;
+
 namespace NATS.Server.Imports;
 
 public sealed class ImportMap
@@ -15,4 +17,23 @@ public sealed class ImportMap
 
         list.Add(si);
     }
+
+    /// <summary>
+    /// Returns the distinct set of source accounts referenced by stream imports.
+    /// Go reference: accounts.go imports.streams — each streamImport has an acc field.
+    /// </summary>
+    public IReadOnlyList<Account> GetStreamImportSourceAccounts()
+    {
+        if (Streams.Count == 0)
+            return [];
+
+        var seen = new HashSet<string>(StringComparer.Ordinal);
+        var result = new List<Account>(Streams.Count);
+        foreach (var si in Streams)
+        {
+            if (seen.Add(si.SourceAccount.Name))
+                result.Add(si.SourceAccount);
+        }
+        return result;
+    }
 }

src/NATS.Server/Imports/ServiceExportInfo.cs

@@ -0,0 +1,12 @@
+namespace NATS.Server.Imports;
+
+/// <summary>
+/// Immutable view of a service export, returned by Account query methods.
+/// IsWildcard is true when the subject contains '*' or '>'.
+/// Go reference: accounts.go serviceExport struct.
+/// </summary>
+public sealed record ServiceExportInfo(
+    string Subject,
+    ServiceResponseType ResponseType,
+    IReadOnlyList<string> ApprovedAccounts,
+    bool IsWildcard);