fix(docs/rerankers): pass rerank=True so examples actually rerank

The reranker examples configured a `reranker` block but called
memory.search(...) without rerank=True. Memory.search() defaults rerank=False
(mem0/memory/main.py:1333) and only reranks 'if rerank and self.reranker'
(:1447), so reranking never ran:

- cohere, zero_entropy, llm, sentence_transformer print result['rerank_score'],
  which doesn't exist unless reranking runs -> KeyError on copy-paste.
- optimization (reranker benchmark) and custom-prompts (reranker-prompt test)
  silently measured un-reranked search.

Add the missing rerank=True flag to every search meant to exercise the reranker.

docs/components/rerankers/custom-prompts.mdx

@@ -193,7 +193,7 @@ for i, prompt in enumerate(prompts):
     config["reranker"]["config"]["scoring_prompt"] = prompt
     memory = Memory.from_config(config)
 
-    results = memory.search("test query", filters={"user_id": "test_user"})
+    results = memory.search("test query", filters={"user_id": "test_user"}, rerank=True)
     print(f"Prompt {i+1} results: {results}")
 ```