fix: repair missing step_type fields in Plan validation (#653)

* fix: resolve issue #650 - repair missing step_type fields in Plan validation

- Add step_type repair logic to validate_and_fix_plan() to auto-infer missing step_type
- Infer as 'research' when need_search=true, 'processing' when need_search=false
- Add explicit CRITICAL REQUIREMENT section to planner.md emphasizing step_type mandatory for every step
- Include validation checklist and examples showing both research and processing steps
- Add 23 comprehensive unit tests for validate_and_fix_plan() covering all scenarios
- Add 4 integration tests specifically for Issue #650 with actual Plan validation
- Prevents Pydantic ValidationError: 'Field required' for missing step_type

* Update tests/unit/graph/test_plan_validation.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update tests/unit/graph/test_plan_validation.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* update the planner.zh_CN.md with recent changes of planner.md

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/graph/nodes.py

@@ -100,6 +100,26 @@ def validate_and_fix_plan(plan: dict, enforce_web_search: bool = False) -> dict:
 
     steps = plan.get("steps", [])
 
+    # ============================================================
+    # SECTION 1: Repair missing step_type fields (Issue #650 fix)
+    # ============================================================
+    for idx, step in enumerate(steps):
+        if not isinstance(step, dict):
+            continue
+        
+        # Check if step_type is missing or empty
+        if "step_type" not in step or not step.get("step_type"):
+            # Infer step_type based on need_search value
+            inferred_type = "research" if step.get("need_search", False) else "processing"
+            step["step_type"] = inferred_type
+            logger.info(
+                f"Repaired missing step_type for step {idx} ({step.get('title', 'Untitled')}): "
+                f"inferred as '{inferred_type}' based on need_search={step.get('need_search', False)}"
+            )
+
+    # ============================================================
+    # SECTION 2: Enforce web search requirements
+    # ============================================================
     if enforce_web_search:
         # Check if any step has need_search=true
         has_search_step = any(step.get("need_search", False) for step in steps)