Memory Store does not allow STM only

## Problem

`createAgentCoreMemoryStores` throws when passed an empty namespaces array, so there's no way to configure short-term-only memory (durable conversation events with no long-term extraction).

Any agent whose AgentCore Memory resource has no long-term strategies hits this at invocation time. The Python SDK supports this shape (long-term retrieval is optional), so the two SDKs diverge.

## Evidence

- The factory rejects an empty namespaces array (still present on latest v0.4.3):
https://github.com/aws/bedrock-agentcore-sdk-typescript/blob/v0.4.3/src/memory/integrations/strands/factory.ts#L130-L133
- Python SDK equivalent makes long-term retrieval optional (short-term works without namespaces):
https://github.com/aws/bedrock-agentcore-sdk-python/blob/v1.9.1/src/bedrock_agentcore/memory/integrations/strands/config.py

Start in src/memory/integrations/strands/factory.ts around lines 130-133, where createAgentCoreMemoryStores rejects an empty namespaces array. Compare the behavior with the linked Python SDK configuration, then verify that an AgentCore Memory resource with no long-term strategies supports short-term memory without throwing.

Written by the indexing model from the issue text.