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/template-parallel-module.yaml

@@ -36,7 +36,7 @@ steps:
 
   - name: verify-parallel
     type: bash
-    pre_condition: 'fileExists("{{parallelOutput}}/bash1.txt")'
+    pre_condition: 'file_exists("{{parallelOutput}}/bash1.txt")'
     command: |
       echo "=== Parallel Steps Verification ==="
       echo "parallel_dir={{parallel_dir}}"