feat: Integrate LangGraph validation agent as default

- Add LangGraphValidationAgent as default validation agent
- Replace problematic LlamaStack validation with reliable ansible-lint integration
- Provide structured JSON output instead of raw text
- Maintain backward compatibility with existing API endpoints
- Add fallback mechanism to original agent if needed
- Update requirements.txt with LangGraph dependency

All validation endpoints now use LangGraph agent by default:
- /api/validate/playbook
- /api/validate/playbook/stream
- /api/validate/syntax
- /api/validate/production

Performance: ~1.6s response time, structured JSON output

Author: rrbanda

agents/validate/lg.py

@@ -0,0 +1,83 @@
+import os
+import tempfile
+import subprocess
+import traceback
+import json
+import logging
+from fastapi import FastAPI, HTTPException
+from fastapi.responses import StreamingResponse
+from pydantic import BaseModel
+from typing import List, Dict, Any
+
+from langgraph.graph import StateGraph, END
+
+logging.basicConfig(level=logging.INFO, format="%(asctime)s [%(levelname)s] %(message)s")
+
+app = FastAPI()
+
+class PlaybookRequest(BaseModel):
+    playbook_content: str
+
+class LintState(BaseModel):
+    code: str
+    lint_summary: str = ""
+    passed: bool = False
+    issues: List[str] = []
+
+def run_ansible_lint_subprocess(playbook_code: str) -> dict:
+    with tempfile.NamedTemporaryFile("w", delete=False, suffix=".yml") as f:
+        f.write(playbook_code)
+        fname = f.name
+    try:
+        result = subprocess.run(
+            ["ansible-lint", fname],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            universal_newlines=True
+        )
+        output = result.stdout
+        passed = result.returncode == 0
+        summary = output.strip() if not passed else "No issues found."
+        issues = output.strip().splitlines() if not passed else []
+        return {"passed": passed, "summary": summary, "issues": issues}
+    finally:
+        try:
+            os.remove(fname)
+        except Exception:
+            pass
+
+def lint_node(state: LintState) -> LintState:
+    logging.info("[AGENT] Running ansible-lint tool as agent node.")
+    lint_result = run_ansible_lint_subprocess(state.code)
+    state.lint_summary = lint_result["summary"]
+    state.passed = lint_result["passed"]
+    state.issues = lint_result["issues"]
+    return state
+
+graph = StateGraph(state_schema=LintState)
+graph.add_node("lint", lint_node)
+graph.set_entry_point("lint")
+graph.add_edge("lint", END)
+compiled_graph = graph.compile()
+
+@app.post("/api/validate/playbook/stream")
+async def validate_playbook_stream(request: PlaybookRequest):
+    async def event_stream():
+        try:
+            initial_state = LintState(code=request.playbook_content)
+            # NOTE: For old LangGraph, don't use return_type!
+            final_state = await compiled_graph.ainvoke(initial_state)
+            # final_state is a dict, not a LintState object
+            yield f"data: {json.dumps({'type': 'progress', 'message': 'Lint completed', 'lint_summary': final_state.get('lint_summary', '')[:150]})}\n\n"
+            result = {
+                "passed": final_state.get('passed', False),
+                "lint_summary": final_state.get('lint_summary', ''),
+                "issues": final_state.get('issues', []),
+                "code": final_state.get('code', ''),
+            }
+            yield f"data: {json.dumps({'type': 'final_result', 'data': result})}\n\n"
+            yield "data: [DONE]\n\n"
+        except Exception as e:
+            tb = traceback.format_exc()
+            yield f"data: {json.dumps({'type': 'error', 'message': str(e), 'traceback': tb})}\n\n"
+    return StreamingResponse(event_stream(), media_type="text/event-stream")