fix: add IndexError guard and error handling test

src/companion/rag/search.py

@@ -67,7 +67,12 @@ class SearchEngine:
         k = top_k or self.default_top_k
 
         try:
-            query_embedding = self.embedder.embed([query])[0]
+            embeddings = self.embedder.embed([query])
+            if not embeddings:
+                raise RuntimeError(
+                    "Failed to generate embedding for query: embedder returned empty result"
+                )
+            query_embedding = embeddings[0]
         except RuntimeError as e:
             raise RuntimeError(f"Failed to generate embedding for query: {e}") from e