Type inconsistency in EventMetadataFilter TypedDict

## Description

In `src/bedrock_agentcore/memory/models/filters.py`, the `EventMetadataFilter` TypedDict defines `operator: OperatorType` (an enum), but the actual value stored is a string.

```python
class EventMetadataFilter(TypedDict):
left: LeftExpression
operator: OperatorType # Type says enum
right: Optional[RightExpression]
```

The `build_expression` method correctly converts the enum to a string:
```python
filter = {"operator": operator.value} # Stores string, not enum
```

## Ramifications

1. **Type hint is misleading** - The TypedDict suggests `operator` holds an `OperatorType` enum, but it actually holds a string like `"EQUALS_TO"`

2. **Potential runtime error** - If a user follows the type hint and passes an enum directly (without using `build_expression`), boto3 will fail to serialize it since enums are not JSON serializable

3. **Type checker inconsistency** - Static type checkers like mypy may produce warnings or miss actual type errors due to this mismatch

Start in src/bedrock_agentcore/memory/models/filters.py by reading EventMetadataFilter alongside build_expression, which converts the operator before storing it. Verify the annotation matches the serialized value, then run the relevant Python tests and type checks; done means the TypedDict and runtime representation agree without misleading callers.

Written by the indexing model from the issue text.