[BUG] v0.2.5: _attach_inline_policy receives dict instead of JSON string for PolicyDocument

## Bug Description

In `v0.2.5`, the `_attach_inline_policy` function in `create_role.py` receives a `dict` for `policy_document`, but the IAM API expects a JSON `string`.

## Error Message

```
Parameter validation failed:
Invalid type for parameter PolicyDocument, value: {...}, type: , valid types:
```

## Steps to Reproduce

1. Run `agentcore deploy` for a new agent
2. The deployment fails when creating the execution role's inline policy

## Root Cause

In `operations/runtime/create_role.py` line 163:

```python
_attach_inline_policy(
iam_client=iam,
role_name=role_name,
policy_name=policy_name,
policy_document=execution_policy, # ← This is a dict, but should be json.dumps(execution_policy)
logger=logger,
)
```

The `_attach_inline_policy` function (line 295) expects `policy_document: str`, but `execution_policy` is a `dict` returned by `validate_rendered_policy()`.

## Suggested Fix

```python
policy_document=json.dumps(execution_policy),
```

## Environment

- OS: macOS (Darwin 24.6.0)
- Python: 3.13
- bedrock-agentcore-starter-toolkit: 0.2.5
- AWS Region: ap-northeast-1

Start in operations/runtime/create_role.py at the _attach_inline_policy call around line 163, then inspect the function around line 295 and validate_rendered_policy's output. Confirm that the IAM request receives PolicyDocument as a JSON string, then run agentcore deploy for a new agent to verify the execution role's inline policy is created successfully.

Written by the indexing model from the issue text.