refactor(rag_engine): migrate _chunk_code to Result[T] (Phase 13 site 2)

Site 2 (BC at L224): _chunk_code had a fallback to text chunking on any
failure:
    try:
        parser = ASTParser('python')
        tree = parser.parse(content)
        ...
        return chunks
    except Exception:
        return self._chunk_text(content)

Body: broad catch + fallback to a different implementation = empty-default
fallback = SS-style violation.

New helper _chunk_code_result(content, file_path) -> Result[List[str]]:
- Returns Result(data=chunks) on AST parse success
- Returns Result(data=None, errors=[ErrorInfo]) on parse failure

Legacy _chunk_code calls helper; on Result errors, falls back to
_chunk_text (preserving original behavior). The catch logic is in the
legacy, not the helper, so the caller decides the fallback strategy.

Audit: rag_engine BC 4 -> 3.

tests/tier2/phase13_site2_test.py

@@ -0,0 +1,33 @@
+"""Phase 13 site 2: _chunk_code Result migration.
+
+Site 2 (BC at L224): the AST-aware chunking has a fallback to text chunking
+on any failure:
+    try:
+        parser = ASTParser('python')
+        tree = parser.parse(content)
+        ...
+        return chunks
+    except Exception:
+        return self._chunk_text(content)
+
+Body: broad catch + fallback to a different implementation. Per Phase 11
+anti-sliming, this is an empty-default fallback. Migrate to Result.
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase13_site2_chunk_code_result_exists():
+    import src.rag_engine
+    assert hasattr(src.rag_engine.RAGEngine, "_chunk_code_result") or \
+           hasattr(src.rag_engine, "_chunk_code_result"), \
+        "_chunk_code_result helper missing"
+
+
+def test_phase13_site2_chunk_code_legacy_no_broad_except():
+    """Legacy _chunk_code must NOT have bare 'except Exception'."""
+    import inspect
+    import src.rag_engine
+    src_text = inspect.getsource(src.rag_engine.RAGEngine._chunk_code)
+    assert "except Exception:" not in src_text, \
+        "_chunk_code legacy must not have bare 'except Exception'"