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/optimization.mdx

@@ -146,7 +146,7 @@ def measure_reranker_performance(config, queries, user_id):
     latencies = []
     for query in queries:
         start_time = time.time()
-        results = memory.search(query, filters={"user_id": user_id})
+        results = memory.search(query, filters={"user_id": user_id}, rerank=True)
         latency = time.time() - start_time
         latencies.append(latency)
 
@@ -218,7 +218,7 @@ async def parallel_search(config, queries, user_id):
 
     # Process multiple queries concurrently
     tasks = [
-        memory.search(query, filters={"user_id": user_id})
+        memory.search(query, filters={"user_id": user_id}, rerank=True)
         for query in queries
     ]