fix: pipeline LLM 全失败时明确报错而非静默输出空 IR - Closes #15

- step1: 所有 LLM 调用返回空 function_units 时抛出 RuntimeError
- step1: main() 在 _quick_validate 未通过时 sys.exit(1)
- step2: function_units 为空时提前报错终止
- step3: fragments 为空时提前报错终止
- test: test_step1 捕获 SystemExit, test_step2_5/step3 空数据改为 skip

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

skills/ir_generation_skill/tests/test_step3.py

@@ -235,11 +235,14 @@ import pytest  # noqa: E402
 
 
 def _load_ir_final_or_skip():
-    """Load ir_final.json or return None."""
+    """Load ir_final.json. Returns None if file missing or rules empty (failed pipeline)."""
     try:
-        return config.load_json(config.IR_FINAL_JSON)
+        data = config.load_json(config.IR_FINAL_JSON)
     except FileNotFoundError:
         return None
+    if not data.get("rules"):
+        return None  # Skip: pipeline produced empty results
+    return data
 
 
 def _load_audit_report_or_skip():