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_step1.py

@@ -376,10 +376,13 @@ def _load_si_and_doc():
     """Try to load semantic_index.json and the input document. Returns (si, doc) or (None, None)."""
     try:
         si = config.load_json(config.SEMANTIC_INDEX_JSON)
-        doc = config.load_input_document()
-        return si, doc
     except FileNotFoundError:
         return None, None
+    try:
+        doc = config.load_input_document()
+    except (FileNotFoundError, SystemExit):
+        return None, None
+    return si, doc
 
 
 def test_step1_unit_ids():