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}}"

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}}"

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}}"

test/testdata/workflows/nested/template-rendering-module.yaml

@@ -26,7 +26,7 @@ steps:
   # 2. Function step with templated function calls
   - name: function-with-templates
     type: function
-    pre_condition: 'fileExists("{{bash_output_path}}")'
+    pre_condition: 'file_exists("{{bash_output_path}}")'
     function: 'log_info("Processing {{Target}} with path {{customPath}}")'
     exports:
       function_result: "processed-{{Target}}"
@@ -50,7 +50,7 @@ steps:
   # 5. Final verification step
   - name: verify-all-templates
     type: bash
-    pre_condition: 'fileExists("{{customPath}}")'
+    pre_condition: 'file_exists("{{customPath}}")'
     command: |
       echo "=== Template Rendering Verification ==="
       echo "bash_output_path={{bash_output_path}}"