fix: correct GraphML namespace URL in GraphMLExporter

The exporter was using http://graphml.graphstruct.org/xmlns which
is not a valid GraphML namespace.  The correct namespace per the
GraphML specification (http://graphml.graphdrawing.org/) is
http://graphml.graphdrawing.org/xmlns.

The wrong namespace caused exported .graphml files to be rejected
by standard parsers in Gephi, Cytoscape, NetworkX, yEd, and igraph
that validate the namespace URI on import.

Fixed all three occurrences in the XML header (xmlns, schemaLocation
namespace, schemaLocation URL) and updated the corresponding test
assertion.

Fixes #20

Author: sauravbhattacharya001

Gvisual/src/gvisual/GraphMLExporter.java

@@ -134,10 +134,10 @@ private String exportToString(List<edge> edgesToExport) {
 
         // XML header
         sb.append("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
-        sb.append("<graphml xmlns=\"http://graphml.graphstruct.org/xmlns\"\n");
+        sb.append("<graphml xmlns=\"http://graphml.graphdrawing.org/xmlns\"\n");
         sb.append("    xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"\n");
-        sb.append("    xsi:schemaLocation=\"http://graphml.graphstruct.org/xmlns\n");
-        sb.append("        http://graphml.graphstruct.org/xmlns/1.0/graphml.xsd\">\n");
+        sb.append("    xsi:schemaLocation=\"http://graphml.graphdrawing.org/xmlns\n");
+        sb.append("        http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd\">\n");
 
         // Key definitions for node and edge attributes
         sb.append("  <!-- Node attribute keys -->\n");

Gvisual/test/gvisual/GraphMLExporterTest.java

@@ -431,7 +431,7 @@ public void testGraphMLNamespace() {
         GraphMLExporter exporter = new GraphMLExporter(graph, allEdges);
         String xml = exporter.exportToString();
 
-        assertTrue(xml.contains("xmlns=\"http://graphml.graphstruct.org/xmlns\""));
+        assertTrue(xml.contains("xmlns=\"http://graphml.graphdrawing.org/xmlns\""));
         assertTrue(xml.contains("xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\""));
     }