fix: preserve array indices in multi-output nodes when removing connections

CRITICAL BUG FIX: Fixed array index corruption in multi-output nodes
(Switch, IF with multiple handlers, Merge) when rewiring connections.

Problem:
- applyRemoveConnection() filtered out empty arrays after removing connections
- This caused indices to shift in multi-output nodes
- Example: Switch.main = [[H0], [H1], [H2]] -> remove H1 -> [[H0], [H2]]
- H2 moved from index 2 to index 1, corrupting workflow structure

Root Cause:
```typescript
// Line 697 - BUGGY CODE:
workflow.connections[node][output] =
  connections.filter(conns => conns.length > 0);
```

Solution:
- Only remove trailing empty arrays
- Preserve intermediate empty arrays to maintain index integrity
- Example: [[H0], [], [H2]] stays [[H0], [], [H2]] not [[H0], [H2]]

Impact:
- Prevents production-breaking workflow corruption
- Fixes rewireConnection operation for multi-output nodes
- Critical for AI agents working with complex workflows

Testing:
- Added integration test for Switch node rewiring with array index verification
- Test creates 4-output Switch node, rewires middle connection
- Verifies indices 0, 2, 3 unchanged after rewiring index 1
- All 137 unit tests + 12 integration tests passing

Discovered by: @agent-n8n-mcp-tester during comprehensive testing
Issue: #272 (Connection Operations - Phase 1)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/services/workflow-diff-engine.ts

@@ -691,12 +691,14 @@ export class WorkflowDiffEngine {
     workflow.connections[sourceNode.name][sourceOutput] = connections.map(conns =>
       conns.filter(conn => conn.node !== targetNode.name)
     );
-    
-    // Clean up empty arrays
-    workflow.connections[sourceNode.name][sourceOutput] = 
-      workflow.connections[sourceNode.name][sourceOutput].filter(conns => conns.length > 0);
-    
-    if (workflow.connections[sourceNode.name][sourceOutput].length === 0) {
+
+    // Remove trailing empty arrays only (preserve intermediate empty arrays to maintain indices)
+    const outputConnections = workflow.connections[sourceNode.name][sourceOutput];
+    while (outputConnections.length > 0 && outputConnections[outputConnections.length - 1].length === 0) {
+      outputConnections.pop();
+    }
+
+    if (outputConnections.length === 0) {
       delete workflow.connections[sourceNode.name][sourceOutput];
     }