fix(memory): pass boto3 session to MemoryClient in AgentCoreMemorySessionManager

[avneetbansal-aws]

Fixes #681.

Problem

`AgentCoreMemorySessionManager.init` constructed its `MemoryClient` without a boto3 session:
```python
self.memory_client = MemoryClient(region_name=region_name) # no session
session = boto_session or boto3.Session(region_name=region_name)
```
Because no session is passed, `MemoryClient.init` builds its `gmcp_client`/`gmdp_client` from a default `boto3.Session()` — so a user-provided `boto_session` (named profile, custom credentials, etc.) is silently discarded on the `MemoryClient` itself. The manager then overwrites those two clients from the correct session, but the `MemoryClient` was still initialized against the wrong one.

Fix

Resolve the session first, then pass it into `MemoryClient`, which already accepts `boto3_session`:
```python
session = boto_session or boto3.Session(region_name=region_name)
self.memory_client = MemoryClient(region_name=region_name, boto3_session=session)
```
The existing `strands-agents` user-agent / `boto_client_config` override that follows is unchanged, so behavior for the overridden clients is identical — the fix just ensures the `MemoryClient` uses the correct session from the start.

Tests

• Updated `test_init_basic` to assert `MemoryClient` receives `boto3_session`.
• Added `test_init_passes_boto_session_to_memory_client` asserting a user-provided `boto_session` reaches `MemoryClient`.
• Full `test_agentcore_memory_session_manager.py` suite passes (189 tests); ruff lint + format clean.