fix: 移除 hardcode 输入文件路径，完善输入验证 - Closes #8

- 移除 _DEFAULT_INPUT 硬编码默认输入文件路径
- INPUT_JSON 仅从 IR_INPUT_JSON 环境变量获取
- load_input_document() 无输入时给出明确错误提示
- 新增 test_no_hardcoded_input_file / test_set_input_file_accepts_none

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

tests/test_config.py

@@ -19,6 +19,33 @@ def test_set_input_file():
         config.set_input_file(original)
 
 
+def test_no_hardcoded_input_file():
+    """INPUT_JSON should not have a hardcoded default — comes from env or None."""
+    # When IR_INPUT_JSON env is not set, INPUT_JSON should be None
+    env_val = os.environ.pop("IR_INPUT_JSON", None)
+    try:
+        import importlib
+        importlib.reload(config)
+        # After reload without env var, INPUT_JSON should be None
+        assert config.INPUT_JSON is None, \
+            f"INPUT_JSON should be None when env not set, got: {config.INPUT_JSON}"
+    finally:
+        if env_val is not None:
+            os.environ["IR_INPUT_JSON"] = env_val
+        importlib.reload(config)
+
+
+def test_set_input_file_accepts_none():
+    """set_input_file(None) should work for resetting."""
+    original = config.INPUT_JSON
+    try:
+        config.set_input_file("/tmp/test.json")
+        config.set_input_file(None)
+        assert config.INPUT_JSON is None
+    finally:
+        config.set_input_file(original)
+
+
 def test_config_constants_exist():
     """Verify all expected path constants are defined."""
     assert config.BASE_DIR