Port AgentCoreMemoryStore (MemoryStore interface) from TypeScript SDK to Python

まだ誰も着手していません。

### Problem Statement

The TypeScript `bedrock-agentcore` SDK (v0.4.1+) ships an `AgentCoreMemoryStore` that implements the Strands `MemoryStore` interface, enabling AgentCore Memory to plug into the `MemoryManager` pipeline with recall (search tool), injection (automatic prompt augmentation), and server-side extraction. It is documented on the Strands integrations page: https://strandsagents.com/docs/integrations/memory-stores/agentcore-memory-store/

The Python `bedrock-agentcore` SDK currently only integrates AgentCore Memory as a session manager (`AgentCoreMemorySessionManager`). There is no `MemoryStore` implementation, so Python developers cannot use AgentCore Memory with `MemoryManager`.

### Proposed Solution

Port the `AgentCoreMemoryStore` from the TypeScript SDK to Python. The integration lives at `bedrock-agentcore/memory/integrations/strands/` in the TS package and maps:

- `search` to AgentCore's `retrieveMemoryRecords`
- `add_messages` to conversation ingestion for server-side extraction
- Per-namespace store creation via a `create_agentcore_memory_stores` factory

Target usage:

```python
from strands import Agent
from strands.memory import MemoryManager
from bedrock_agentcore.memory.integrations.strands.memory_store import (
AgentCoreMemoryStore,
create_agentcore_memory_stores,
)

stores = create_agentcore_memory_stores(
memory_id="mem-abc",
actor_id="user-123",
session_id="session-1",
namespaces=[
{"namespace": "/facts/{actorId}", "writable": True},
{"namespace": "/preferences/{actorId}"},
],
extraction=True,
)

agent = Agent(memory_manager=MemoryManager(stores=stores))
```

### Use Case

Python agents that want to use AgentCore Memory as a long-term memory backend through the standard `MemoryManager` pipeline, with injection, recall tools, and extraction.

### Additional Context

- TypeScript implementation: https://github.com/aws/bedrock-agentcore-sdk-typescript/tree/main/src/memory/integrations/strands
- Strands integration page (TS only): https://strandsagents.com/docs/integrations/memory-stores/agentcore-memory-store/
- The existing Python `AgentCoreMemorySessionManager` would remain for session persistence use cases. The `MemoryStore` is complementary.

まず bedrock-agentcore/memory/integrations/strands 配下の TypeScript 実装を読み、Python AgentCoreMemorySessionManager と比較します。issue で説明されている Python memory_store.py のエントリポイントと factory を追加し、search と add_messages を AgentCore Memory にマッピングします。Python ユーザーが作成された store を Strands MemoryManager に渡して recall、injection、extraction に利用できれば完了です。

索引モデルが issue の本文から書いたものです。