fix(rag): handle None metadata in get_all_indexed_paths and non-empty numpy in dim check

Two bugs in src/rag_engine.py were causing 'NoneType object has no attribute get'
in the live_gui RAG tests (test_rag_phase4_final_verify,
test_rag_phase4_stress):

1. _validate_collection_dim_result:148
   Old:  if not embeddings or len(embeddings) == 0:
   New:  if embeddings is None or len(embeddings) == 0:
   The 'if not embeddings' check raises ValueError('The truth value of an
   array with more than one element is ambiguous. Use a.any() or a.all()')
   when 'embeddings' is a non-empty numpy array (which is the normal case
   after documents are upserted). The exception is caught by the outer
   'except Exception' which returns a non-ok Result, causing __init__ to
   set self.collection = None. Subsequent 'get_all_indexed_paths()' then
   fails with 'NoneType has no attribute get' on self.collection.get().

2. get_all_indexed_paths:334
   Old:  return list(set(m.get('path') for m in res['metadatas'] if m.get('path')))
   New:  return list(set(m['path'] for m in res['metadatas'] if m is not None and m.get('path')))
   When chromadb returns 'metadatas=[None, ...]' (documents upserted
   without metadata), 'm.get('path')' fails with AttributeError on the
   first None element. Adds 'm is not None' guard.

Both fixes are defensive: the conditions that trigger them (orphan docs
without metadata, non-empty embeddings arrays) are normal valid
states that the old code couldn't handle.

New file: tests/test_rag_sync_none_error.py
   3 unit tests covering both bugs:
   - test_dim_check_does_not_raise_on_non_empty_ndarray
   - test_get_all_indexed_paths_handles_none_metadata
   - test_get_all_indexed_paths_returns_paths_with_metadata

Verified:
- 3/3 focused tests pass
- test_rag_phase4_final_verify.py::test_phase4_final_verify PASSES (was failing)
- test_rag_phase4_stress.py::test_rag_large_codebase_verification_sim PASSES (was failing)
- test_rag_visual_sim.py::test_rag_full_lifecycle_sim PASSES (still passing)

src/rag_engine.py

@@ -147,7 +147,7 @@ class RAGEngine:
    if not res:
     return Result(data=None)
    embeddings = res.get("embeddings") if isinstance(res, dict) else None
-   if not embeddings or len(embeddings) == 0:
+   if embeddings is None or len(embeddings) == 0:
     return Result(data=None)
    existing_dim = len(embeddings[0])
    expected_dim = len(self.embedding_provider.embed(["__rag_dim_check__"])[0])
@@ -331,7 +331,7 @@ class RAGEngine:
   res = self.collection.get(include=["metadatas"])
   if not res or not res["metadatas"]:
    return []
-  return list(set(m.get("path") for m in res["metadatas"] if m.get("path")))
+  return list(set(m["path"] for m in res["metadatas"] if m is not None and m.get("path")))
 
  def delete_documents_by_path(self, file_paths: List[str]):
   if not self.config.enabled or self.collection == "mock":