feat: add LLM step executor with vision and tool support, event workflow system, and inheritance

- Add LLM executor supporting OpenAI vision, tool calling, embeddings, and structured outputs
- Introduce event emitter/receiver workflows with deduplication and filtering (generate_event functions)
- Add workflow extends/override system enabling inheritance chains and step merge modes
- Update function naming to snake_case across all testdata (fileExists→file_exists, etc.)
- Add comprehensive test fixtures for linter, events, CDN, step dependencies, and extends workflows

test/testdata/workflows/nested/nested-module-2.yaml

@@ -24,7 +24,7 @@ steps:
 
   - name: verify-exports-and-params
     type: bash
-    pre_condition: 'fileExists("{{anotherParam3as1}}")'
+    pre_condition: 'file_exists("{{anotherParam3as1}}")'
     command: |
       echo "Module 2: paramFromFlowFile={{paramFromFlowFile}}"
       echo "Module 2: module1_completed={{module1_completed}}"