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-foreach-module.yaml

@@ -21,7 +21,7 @@ steps:
   # Foreach with templated input path
   - name: foreach-with-templates
     type: foreach
-    pre_condition: 'fileExists("{{inputFile}}")'
+    pre_condition: 'file_exists("{{inputFile}}")'
     input: "{{inputFile}}"
     variable: item
     threads: 2
@@ -34,7 +34,7 @@ steps:
   # Verify results
   - name: verify-foreach
     type: bash
-    pre_condition: 'fileExists("{{outputDir}}/item1.txt")'
+    pre_condition: 'file_exists("{{outputDir}}/item1.txt")'
     command: |
       echo "=== Foreach Verification ==="
       echo "Foreach completed for {{Target}}"