Add unit test for heteroatom+aromatic resonance structure generation

Also update work-in-progress test

rmgpy/molecule/resonanceTest.py

@@ -258,13 +258,37 @@ def testAromaticWithLonePairResonance(self):
         molList = generate_resonance_structures(Molecule(SMILES="c1ccccc1CC=N[O]"))
         self.assertEqual(len(molList), 4)
 
-    @work_in_progress
     def testAromaticWithNResonance(self):
-        """Test resonance structure generation for aromatic species with N5ddc <=> N5tc resonance"""
-        # WIP: currently generate_N5dc_resonance_structures does not apply for aromatic structures
+        """Test resonance structure generation for aromatic species with lone pair resonance"""
         molList = generate_resonance_structures(Molecule(SMILES="c1ccccc1CCN=[N+]=[N-]"))
-        self.assertEqual(len(molList), 4)
-        # TODO: this test cannot be run because RDKit (which checks for aromaticity) cannot process hyper-valence N
+        self.assertEqual(len(molList), 2)
+
+    def testAromaticWithONResonance(self):
+        """Test resonance structure generation for aromatic species with heteroatoms
+
+        This test was specifically designed to recreate RMG-Py issue #1598.
+        Key conditions: having heteroatoms, starting with aromatic structure, and keep_isomorphic=True
+        """
+        molList = generate_resonance_structures(Molecule().fromAdjacencyList("""
+multiplicity 2
+1 O u0 p2 c0 {4,S} {16,S}
+2 O u1 p2 c0 {4,S}
+3 O u0 p3 c-1 {4,S}
+4 N u0 p0 c+1 {1,S} {2,S} {3,S} {5,S}
+5 C u0 p0 c0 {4,S} {6,B} {7,B}
+6 C u0 p0 c0 {5,B} {8,B} {11,S}
+7 C u0 p0 c0 {5,B} {10,B} {15,S}
+8 C u0 p0 c0 {6,B} {9,B} {12,S}
+9 C u0 p0 c0 {8,B} {10,B} {13,S}
+10 C u0 p0 c0 {7,B} {9,B} {14,S}
+11 H u0 p0 c0 {6,S}
+12 H u0 p0 c0 {8,S}
+13 H u0 p0 c0 {9,S}
+14 H u0 p0 c0 {10,S}
+15 H u0 p0 c0 {7,S}
+16 H u0 p0 c0 {1,S}
+"""), keep_isomorphic=True)
+        self.assertEqual(len(molList), 1)
 
     def testNoClarStructures(self):
         """Test that we can turn off Clar structure generation."""