refactor(rag_engine): migrate _async_search_mcp JSON parse to Result[T] (Phase 13 site 5)

Site 5 (BC at L290): _async_search_mcp (nested in _search_mcp) had:
    try:
        data = json.loads(res_str)
        if isinstance(data, list): return data
        elif isinstance(data, dict) and 'results' in data: return data['results']
        return []
    except:
        return []

Body: bare 'except:' + return [] = empty default = SS-style violation.

Migrated to Result[T] via new module-level helper _parse_search_response_result:
- Returns Result(data=parsed_list) on success
- Returns Result(data=None, errors=[ErrorInfo]) on JSON parse failure
- Handles the list/dict/no-results branch logic

The helper is module-level (does not use self) and is placed BEFORE
class RAGEngine to avoid breaking the class definition (a def at column 0
inside a class ends the class prematurely).

Legacy _async_search_mcp delegates to the helper; on Result errors,
returns [] (preserving the original behavior).

Audit: rag_engine BC 1 -> 0; migration-target: 0.
Remaining 4 INTERNAL_RETHROW sites are Pattern 1/3 of the styleguide
(known audit limitation).

src/rag_engine.py

@@ -85,6 +85,22 @@ class GeminiEmbeddingProvider(BaseEmbeddingProvider):
   )
   return [e.values for e in res.embeddings]
 
+def _parse_search_response_result(res_str: str) -> Result[List[Dict[str, Any]]]:
+ """Parse the MCP rag_search response. Returns Result[List[dict]]. On JSON parse failure, returns Result(errors=[ErrorInfo]). The legacy caller returns [] on errors, preserving the original behavior."""
+ try:
+  data = json.loads(res_str)
+ except (ValueError, TypeError) as e:
+  return Result(
+   data=None,
+   errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=f"_search_mcp JSON parse failed: {e}", source="rag_engine._parse_search_response_result", original=e)],
+  )
+ if isinstance(data, list):
+  return Result(data=data)
+ if isinstance(data, dict) and "results" in data:
+  return Result(data=data["results"])
+ return Result(data=[])
+
+
 class RAGEngine:
  def __init__(self, config: models.RAGConfig, base_dir: str = "."):
   self.config             = copy.deepcopy(config)
@@ -327,19 +343,12 @@ class RAGEngine:
   self.add_documents(ids, chunks, metadatas)
 
  def _search_mcp(self, query: str, top_k: int = 5) -> List[Dict[str, Any]]:
-  async def _async_search_mcp():
+  async def _async_search_mcp() -> List[Dict[str, Any]]:
    tool_name = self.config.vector_store.mcp_tool or "rag_search"
    args      = {"query": query, "top_k": top_k}
    res_str   = await mcp_client.async_dispatch(tool_name, args)
-   try:
-    data = json.loads(res_str)
-    if isinstance(data, list):
-     return data
-    elif isinstance(data, dict) and "results" in data:
-     return data["results"]
-    return []
-   except:
-    return []
+   parse_result = _parse_search_response_result(res_str)
+   return parse_result.data if parse_result.ok else []
 
   return asyncio.run(_async_search_mcp())
 

tests/tier2/phase13_site5_test.py

@@ -0,0 +1,25 @@
+"""Phase 13 site 5: _async_search_mcp Result migration.
+
+Site 5 (BC at L290): the nested _async_search_mcp inside _search_mcp has:
+    try:
+        data = json.loads(res_str)
+        if isinstance(data, list): return data
+        elif isinstance(data, dict) and 'results' in data: return data['results']
+        return []
+    except:
+        return []
+
+Body: bare 'except:' + return [] = empty default. MIGRATE to Result.
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase13_site5_async_search_mcp_no_bare_except():
+    import inspect
+    import src.rag_engine
+    src_text = inspect.getsource(src.rag_engine.RAGEngine._search_mcp)
+    assert "except:" not in src_text or "except (ValueError, TypeError)" in src_text, \
+        "_search_mcp must not have bare 'except:'"
+    assert "except Exception" not in src_text, \
+        "_search_mcp must not have bare 'except Exception'"