refactor(parsers): 提取 _validate_path helper（T7 架构审查整改，I8 DRY）

- source_aggregator.py 三处重复校验（扩展名白名单 + 存在性）提取为
  _validate_path(path, allowed_exts) -> Path，保留 ValueError/FileNotFoundError
  语义与错误信息，parse 三分支改用 helper
- 外部契约不变：未知扩展名→ValueError、不存在→FileNotFoundError
- 新增 3 用例直接测 helper
- TDD: RED（helper 不存在）→ GREEN（聚焦 13 passed）→ 全量 248 passed / 100.00%（1356 stmts/332 br）

tests/test_source_aggregator.py

@@ -1,4 +1,5 @@
 import pytest
+from pathlib import Path
 
 from genesis.data_models import StructuredSource
 from genesis.parsers.source_aggregator import SourceParser
@@ -96,3 +97,31 @@ def test_parse_extensionless_rule(tmp_path):
     bad.write_text("hello", encoding="utf-8")
     with pytest.raises(ValueError, match="无扩展名"):
         SourceParser().parse(rule_paths=[bad])
+
+
+# ---------- T7: DRY _validate_path helper（I8） ----------
+
+def test_validate_path_rejects_bad_extension(tmp_path):
+    from genesis.parsers.source_aggregator import _validate_path
+
+    bad = tmp_path / "note.txt"
+    bad.write_text("x", encoding="utf-8")
+    with pytest.raises(ValueError, match="不支持的文件类型"):
+        _validate_path(bad, (".xlsx",))
+
+
+def test_validate_path_rejects_missing_file(tmp_path):
+    from genesis.parsers.source_aggregator import _validate_path
+
+    with pytest.raises(FileNotFoundError):
+        _validate_path(tmp_path / "missing.xlsx", (".xlsx",))
+
+
+def test_validate_path_returns_resolved_path(tmp_path):
+    from genesis.parsers.source_aggregator import _validate_path
+
+    good = tmp_path / "ok.xlsx"
+    good.write_bytes(b"x")
+    result = _validate_path(str(good), (".xlsx",))
+    assert isinstance(result, Path)
+    assert result.suffix.lower() == ".xlsx"