Expired AWS Login credentials are reported as invalid initialize parameters

## Summary

When AWS Login credentials require reauthentication, `mcp-proxy-for-aws` converts the underlying `LoginRefreshRequired` failure during initialization into:

`JSON-RPC error: -32602: Invalid request parameters("")`

The initialize request itself is valid, so MCP client users receive a misleading diagnostic and cannot identify that `aws login` is required.

## Environment

- mcp-proxy-for-aws: 1.6.0
- Transport: stdio proxy to `https://aws-mcp.us-east-1.api.aws/mcp`
- AWS credential source: AWS CLI `aws login`

## Reproduction

1. Configure the proxy normally with AWS Login credentials.
2. Let the AWS Login refresh token expire or otherwise reach a state that requires `aws login`.
3. Send a valid MCP initialize request.

Example request:

```json
{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"diagnostic","version":"1"}}}
```

## Actual result

```json
{"jsonrpc":"2.0","id":1,"error":{"code":-32602,"message":"Invalid request parameters","data":""}}
```

With DEBUG logging enabled, the underlying failure is:

```text
botocore.exceptions.LoginRefreshRequired:
Your session has expired or credentials have changed.
Please reauthenticate using 'aws login'.
```

Running `aws login` restores the same initialize handshake.

## Expected result

Preserve an identifiable authentication error, or emit a clear authentication diagnostic, instead of reporting that valid initialize parameters are invalid.

## Additional context

This affects MCP clients including Codex and Cursor because they only display the returned `-32602` initialize error by default.

Start by tracing the MCP initialize request handling and how botocore.exceptions.LoginRefreshRequired is converted into the JSON-RPC error. Reproduce with expired AWS Login credentials and DEBUG logging, then verify that a valid initialize request produces an identifiable authentication diagnostic rather than invalid request parameters.

Written by the indexing model from the issue text.