Use AWS MemoryCreated waiter in MemoryClient._wait_for_memory_active()

## Summary

Replace manual polling in `MemoryClient._wait_for_memory_active()` with the AWS `memory_created` waiter for more efficient waiting.

## Current Implementation

The current `_wait_for_memory_active()` method uses manual polling with a default 10-second interval:

```python
def _wait_for_memory_active(self, memory_id: str, max_wait: int, poll_interval: int) -> Dict[str, Any]:
start_time = time.time()
while time.time() - start_time < max_wait:
status = self.get_memory_status(memory_id)
if status == MemoryStatus.ACTIVE.value:
return memory
elif status == MemoryStatus.FAILED.value:
raise RuntimeError(...)
time.sleep(poll_interval) # Default: 10 seconds
raise TimeoutError(...)
```

## Proposed Implementation

Use the `memory_created` waiter (confirmed available in boto3):

```python
def _wait_for_memory_active(self, memory_id: str, max_wait: int = 300, poll_interval: int = 2) -> Dict[str, Any]:
try:
waiter = self.gmcp_client.get_waiter('memory_created')
waiter.wait(
memoryId=memory_id,
WaiterConfig={
'Delay': poll_interval,
'MaxAttempts': max_wait // poll_interval
}
)
# Get final memory state
response = self.gmcp_client.get_memory(memoryId=memory_id)
return self._normalize_memory_response(response["memory"])
except Exception:
# Fallback to manual polling for backwards compatibility
return self._wait_for_memory_active_polling(memory_id, max_wait, poll_interval)
```

## Benefits

| Aspect | Manual Polling | AWS Waiter |
|--------|---------------|------------|
| Default poll interval | 10s | 2s |
| Error handling | Custom | AWS-managed |
| Retry logic | Custom | Built-in |
| Maintenance | SDK team | AWS/botocore |

## Affected Methods

This change would improve the following methods that use `_wait_for_memory_active()`:
- `create_memory_and_wait()`
- `add_*_strategy_and_wait()` methods
- `update_memory_strategies_and_wait()`

## Backwards Compatibility

Include fallback to manual polling if:
- Waiter is not available in older boto3 versions
- Waiter fails unexpectedly

## Notes

- The `memory_created` waiter is confirmed available: `client.waiter_names` returns `['memory_created']`
- No `memory_deleted` waiter exists, so `delete_memory_and_wait()` must continue using manual polling