fix: improve rewireConnection validation to check specific sourceIndex

Addresses code review feedback - rewireConnection now validates that a
connection exists at the SPECIFIC sourceIndex, not just at any index.

Problem:
- Previous validation checked if connection existed at ANY index
- Could cause confusing runtime errors instead of clear validation errors
- Example: Connection exists at index 0, but rewireConnection uses index 1

Fix:
- Resolve smart parameters to get actual sourceIndex
- Validate connection exists at connections[sourceOutput][sourceIndex]
- Provide clear error message with specific index

Impact:
- Better validation error messages
- Prevents confusing runtime errors
- Clearer feedback to AI agents

Code Review: High priority fix from @agent-code-reviewer

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

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

src/services/workflow-diff-engine.ts

@@ -484,20 +484,22 @@ export class WorkflowDiffEngine {
     }
 
     // Resolve smart parameters (branch, case) before validating connections
-    const { sourceOutput } = this.resolveSmartParameters(workflow, operation);
+    const { sourceOutput, sourceIndex } = this.resolveSmartParameters(workflow, operation);
 
-    // Validate that connection from source to "from" exists
+    // Validate that connection from source to "from" exists at the specific index
     const connections = workflow.connections[sourceNode.name]?.[sourceOutput];
     if (!connections) {
       return `No connections found from "${sourceNode.name}" on output "${sourceOutput}"`;
     }
 
-    const hasConnection = connections.some(conns =>
-      conns.some(c => c.node === fromNode.name)
-    );
+    if (!connections[sourceIndex]) {
+      return `No connections found from "${sourceNode.name}" on output "${sourceOutput}" at index ${sourceIndex}`;
+    }
+
+    const hasConnection = connections[sourceIndex].some(c => c.node === fromNode.name);
 
     if (!hasConnection) {
-      return `No connection exists from "${sourceNode.name}" to "${fromNode.name}" on output "${sourceOutput}"`;
+      return `No connection exists from "${sourceNode.name}" to "${fromNode.name}" on output "${sourceOutput}" at index ${sourceIndex}"`;
     }
 
     return null;