bug(bedrockagentcore): Memory L2 construct does not set defaultChild, apply_removal_policy fails

### Describe the bug

The `Memory` L2 construct in `aws_bedrockagentcore` does not set `defaultChild` on the construct node. This causes `apply_removal_policy()` to fail with:

```
CannotApplyRemovalPolicy: Cannot apply RemovalPolicy: no child or not a CfnResource. Apply the removal policy on the CfnResource directly.
```

### Expected Behavior

`memory.apply_removal_policy(RemovalPolicy.RETAIN)` should work, like it does on other L2 constructs (e.g., `s3.Bucket`, `dynamodb.Table`).

### Current Behavior

The `Memory` L2 creates a `CfnMemory` child named `"Memory"` and a `Role` child named `"ServiceRole"`, but does not assign the `CfnMemory` as the `default_child`. The inherited `Resource.apply_removal_policy()` then fails because it relies on `self.node.default_child` being set.

### Workaround

```python
from typing import cast
from aws_cdk import CfnResource

cfn_memory = cast(CfnResource, memory.node.find_child("Memory"))
cfn_memory.apply_removal_policy(RemovalPolicy.RETAIN)
```

### Reproduction Steps

```python
import aws_cdk as cdk
from aws_cdk import aws_bedrockagentcore as agentcore, RemovalPolicy

app = cdk.App()
stack = cdk.Stack(app, "Test")

memory = agentcore.Memory(stack, "M",
memory_name="test",
expiration_duration=cdk.Duration.days(30),
memory_strategies=[agentcore.MemoryStrategy.using_built_in_semantic()],
)

# This fails:
memory.apply_removal_policy(RemovalPolicy.RETAIN)
```

### Additional context

- `aws-cdk-lib` version: 2.261.0
- `node.default_child` is `None`
- The `CfnMemory` resource is at `node.find_child("Memory")`

### CDK CLI Version

2.261.0

### Framework Version

2.261.0

### Node.js Version

20.x

### OS

Amazon Linux 2023

### Language

Python

### Language Version

3.12

### Other information

_No response_

Start at the aws_bedrockagentcore Memory L2 construct and its CfnMemory child, then inspect how apply_removal_policy resolves node.default_child. Reproduce the provided RETAIN case and add coverage showing that the CfnMemory child is used as the default child.

Written by the indexing model from the issue text.