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_step2_5.py

@@ -160,6 +160,8 @@ def test_step2_5_path_enumeration():
         path_data = config.load_json(config.PATH_ENUM_JSON)
     except FileNotFoundError:
         pytest.skip("path_enumeration.json not found — run step2_5_branch_coverage.py first")
+    if path_data.get("total_paths", 0) == 0:
+        pytest.skip("path_enumeration.json has 0 paths — pipeline may have failed upstream")
     errors = check_path_enumeration(path_data)
     assert not errors, f"path enumeration errors: {errors}"