feat(validator): detect IF/Switch/Filter conditional branch fan-out misuse

Add CONDITIONAL_BRANCH_FANOUT warning when conditional nodes have all
connections on main[0] with higher outputs empty, indicating both
branches execute together instead of being split by condition.

Extract getShortNodeType() and getConditionalOutputInfo() helpers to
deduplicate conditional node detection logic.

Conceived by Romuald Czlonkowski - https://www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dist/services/workflow-validator.d.ts

@@ -85,7 +85,12 @@ export declare class WorkflowValidator {
     private validateErrorOutputConfiguration;
     private validateAIToolConnection;
     private validateAIToolSource;
+    private getNodeOutputTypes;
+    private validateNotAISubNode;
+    private getShortNodeType;
+    private getConditionalOutputInfo;
     private validateOutputIndexBounds;
+    private validateConditionalBranchUsage;
     private validateInputIndexBounds;
     private flagOrphanedNodes;
     private validateTriggerReachability;