fix(rag): wipe chroma dir on dim mismatch instead of delete_collection

When the existing collection has embeddings from a different
embedding provider (e.g. Gemini 3072-dim vs local 384-dim), the
prior approach of calling client.delete_collection() fails with
'RustBindingsAPI object has no attribute bindings' in chromadb 1.5.x
when the underlying state is corrupted. rmtree is reliable and
re-creates a fresh empty collection.

Also fixes:
- 'The truth value of an empty array is ambiguous' on numpy 2.x
  by using try/except around len() instead of truthiness check
- WinError 32 on rmtree by closing the chroma client first

Verified: tests/test_rag_phase4_final_verify.py passes in isolation
in 7.75s after this fix. The test still fails in batch context due
to a separate io_pool race condition (multiple _sync_rag_engine
calls collide when the test sets rag_enabled, rag_source, and
rag_emb_provider in sequence). The race is in app_controller.py
and is out of scope for this defensive fix.

Note: tests/test_rag_engine.py has explicit unit tests for
test_rag_collection_dim_mismatch_recreates_collection and
test_rag_collection_dim_match_preserves_collection which
exercise this code path.

src/rag_engine.py

@@ -129,36 +129,72 @@ class RAGEngine:
   Detect dimension mismatch between an existing collection's vectors and
   the current embedding provider's output. When mismatched (e.g. the user
   switched from Gemini 3072-dim to local 384-dim, or vice versa), the
-  collection is deleted and recreated empty so the next index pass
-  populates it with the correct dim. Prevents silent corruption that
-  would later surface as a search error ("Collection expecting
-  embedding with dimension of X, got Y") and hang live_gui tests.
+  collection is wiped at the directory level (not via delete_collection,
+  which can fail on corrupted state in chromadb 1.5.x with
+  "RustBindingsAPI object has no attribute bindings") so the next
+  index pass populates it with the correct dim. Prevents silent
+  corruption that would later surface as a search error
+  ("Collection expecting embedding with dimension of X, got Y") and
+  hang live_gui tests.
   [C: tests/test_rag_engine.py:test_rag_collection_dim_mismatch_recreates_collection, tests/test_rag_engine.py:test_rag_collection_dim_match_preserves_collection]
   """
   if self.collection is None or self.collection == "mock" or self.embedding_provider is None:
    return
   try:
    res = self.collection.get(limit=1, include=["embeddings"])
-   if not res:
-    return
-   embeddings = res.get("embeddings") if isinstance(res, dict) else None
-   if not embeddings or len(embeddings) == 0:
-    return
-   existing_dim = len(embeddings[0])
-   expected_dim = len(self.embedding_provider.embed(["__rag_dim_check__"])[0])
-   if existing_dim == expected_dim:
-    return
-   sys.stderr.write(
-    f"RAG: Collection '{self.collection.name}' dim mismatch "
-    f"(existing={existing_dim}, expected={expected_dim}). "
-    f"Recreating collection to prevent silent corruption.\n"
-   )
-   sys.stderr.flush()
-   self.client.delete_collection(self.collection.name)
-   self.collection = self.client.get_or_create_collection(name=self.collection.name)
   except Exception as e:
-   sys.stderr.write(f"RAG: Failed to validate collection dim: {e}\n")
+   sys.stderr.write(f"RAG: Failed to read collection for dim check: {e}\n")
    sys.stderr.flush()
+   return
+  if not res:
+   return
+  embeddings = res.get("embeddings") if isinstance(res, dict) else None
+  if embeddings is None:
+   return
+  # Use numpy-safe emptiness check (numpy 2.x disallows truthiness on empty arrays)
+  try:
+   if len(embeddings) == 0:
+    return
+  except TypeError:
+   return
+  existing_dim = len(embeddings[0])
+  try:
+   expected_dim = len(self.embedding_provider.embed(["__rag_dim_check__"])[0])
+  except Exception as e:
+   sys.stderr.write(f"RAG: Failed to compute expected dim: {e}\n")
+   sys.stderr.flush()
+   return
+  if existing_dim == expected_dim:
+   return
+  sys.stderr.write(
+   f"RAG: Collection '{self.collection.name}' dim mismatch "
+   f"(existing={existing_dim}, expected={expected_dim}). "
+   f"Wiping chroma dir to prevent silent corruption.\n"
+  )
+  sys.stderr.flush()
+  # Wipe the entire chroma dir (not via delete_collection which
+  # fails on corrupted state in chromadb 1.5.x with
+  # "RustBindingsAPI object has no attribute bindings"). Rmtree is
+  # reliable and re-creates a fresh empty collection.
+  import shutil as _shutil
+  # Close the chroma client first to release file handles. Without
+  # this, rmtree fails with WinError 32 on Windows.
+  try:
+   if hasattr(self, 'client') and self.client and self.client != "mock":
+    self.client.close()
+  except Exception:
+   pass
+  self.client = None
+  self.collection = None
+  if hasattr(self, 'base_dir') and self.base_dir:
+   db_path = os.path.abspath(os.path.join(self.base_dir, ".slop_cache", f"chroma_{self.config.vector_store.collection_name}"))
+   if os.path.isdir(db_path):
+    try:
+     _shutil.rmtree(db_path)
+    except Exception as e:
+     sys.stderr.write(f"RAG: Failed to wipe chroma dir: {e}\n")
+     sys.stderr.flush()
+  self._init_vector_store()
 
  def is_empty(self) -> bool:
   if not self.config.enabled: