`aws configure agent-toolkit` fails to configure MCP server on non-English Windows environments

### Describe the bug

## Environment

* Japanese Windows 11
* AWS CLI v2.35.10
* Claude Code v2.1.187

Running `aws configure agent-toolkit` on Japanese Windows 11 to configure an MCP server results in an error.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

The MCP server configuration is saved without errors.

### Current Behavior

The error when configuring the Claude Code MCP server is as follows.

```powershell
# Use Claude Code
PS C:\> aws configure agent-toolkit --region us-east-1

Detecting installed AI coding agents...
✓ Claude Code — C:\Users\xxxxxxxx/.claude/skills
✗ Cline — ~/.cline/skills (not found)
✗ Codex — ~/.agents/skills (not found)
✗ Cursor — ~/.cursor/skills (not found)
✗ Gemini CLI — ~/.agents/skills (not found)
✓ Kiro — C:\Users\xxxxxxxx/.kiro/skills
✗ OpenClaw — ~/.openclaw/skills (not found)
✗ OpenCode — ~/.agents/skills (not found)
✗ Pi — ~/.pi/agent/skills (not found)
✗ Windsurf — ~/.agents/skills (not found)

Select agents to configure (space to toggle, up/down arrows to navigate, enter to confirm):
[x] Claude Code — C:\Users\xxxxxxxx/.claude/skills
[ ] Kiro — C:\Users\xxxxxxxx/.kiro/skills
```

Attempting to configure the MCP server produces the following error.

```powershell
Fetching default AWS skills...
Found: amazon-bedrock, aws-billing-and-cost-management, aws-blocks, aws-cdk, aws-cloudformation, aws-containers, aws-iam, aws-messaging-and-streaming, aws-observability, aws-sdk-js-v3-usage, aws-sdk-python-usage, aws-sdk-swift-usage, aws-serverless, signing-in-to-aws

Install 14 default AWS skills? [Y/n]: n

Configure AWS MCP server connection? [Y/n]: y

AWS MCP server configured for:

aws: [ERROR]: 'cp932' codec can't decode byte 0x94 in position 17949: illegal multibyte sequence
```

Full traceback with `--debug`.

```powershell
Configure AWS MCP server connection? [Y/n]: 2026-06-26 17:23:42,835 - MainThread - awscli.compat - DEBUG - readline module not available
y

AWS MCP server configured for:
2026-06-26 17:23:44,534 - MainThread - awscli.clidriver - DEBUG - Exception caught in main()
Traceback (most recent call last):
File "awscli\clidriver.py", line 602, in main
File "awscli\lazy.py", line 43, in __call__
File "awscli\customizations\commands.py", line 153, in __call__
File "awscli\customizations\commands.py", line 207, in __call__
File "awscli\customizations\agenttoolkit\configure.py", line 126, in _run_main
File "awscli\customizations\agenttoolkit\configure.py", line 206, in _configure_mcp
File "awscli\customizations\agenttoolkit\agents.py", line 225, in configure_mcp_server
File "awscli\customizations\agenttoolkit\agents.py", line 255, in _read_mcp_config
File "json\__init__.py", line 298, in load
UnicodeDecodeError: 'cp932' codec can't decode byte 0x94 in position 17949: illegal multibyte sequence

aws: [ERROR]: 'cp932' codec can't decode byte 0x94 in position 17949: illegal multibyte sequence
```

### Reproduction Steps

Configure the MCP server by running the `aws configure agent-toolkit` command while retaining an agent configuration file that contains characters not supported by CP932 in a Japanese Windows environment.

### Possible Solution

## Suspected root cause

The MCP server configuration file (`~/.claude.json`) contains an **em-dash** (U+2014, 0xE28094 in UTF-8).

The `_read_mcp_config` and `_write_mcp_config` methods handle reading and writing the configuration file, but do not specify an encoding when opening the file. On Japanese Windows, Python defaults to CP932, which cannot decode the UTF-8 encoded em-dash, causing the error.

https://github.com/aws/aws-cli/blob/2.35.10/awscli/customizations/agenttoolkit/agents.py#L250-L267

## Proposed Fix

* Use `compat_open` instead of `open` when opening the file, or
* Explicitly open the file with UTF-8 encoding

Since the configuration files for each agent are expected to be UTF-8 encoded (this hasn't been explicitly verified, but it is highly likely), hardcoding UTF-8 may not cause any issues.
Alternatively, I think it would be fine to include the file encoding settings in [AGENT_CONFIGS](https://github.com/aws/aws-cli/blob/2.35.10/awscli/customizations/agenttoolkit/agents.py#L282-L387).

### Additional Information/Context

A similar issue may occur not only in the Japanese Windows but also in non-English Windows environments.

### CLI version used

2.35.10

### Environment details (OS name and version, etc.)

Japanese Windows 11