400 "Invalid MCP request header" when X-MCP-Transport header is sent to Aiven API

Body:

## Description

All API requests from `mcp-aiven@0.1.5` (stdio transport) fail with:

Aiven API Error (400): Invalid MCP request header [GET https://api.aiven.io/v1/project] (https://api.aiven.io/v1/project])

The Aiven API rejects requests containing the `X-MCP-Transport` header (and possibly `X-MCP-Tool-Name` / `X-MCP-Client`) introduced in PR #40.

## Steps to reproduce

1. Configure `mcp-aiven` with a valid `AIVEN_TOKEN` (stdio mode)
2. Call any tool (e.g. `aiven_project_list`)
3. Request fails with 400

## Verification

Direct API call with the same token works fine:

```bash
curl -s -o /dev/null -w "%{http_code}" \
-H "Authorization: Bearer $AIVEN_TOKEN" \
https://api.aiven.io/v1/project
# Returns 200
```

Root cause

client.js buildHeaders() adds X-MCP-Transport: stdio (and optionally X-MCP-Tool-Name, X-MCP-Client) to every request. The Aiven API now rejects these as invalid headers.

Workaround

Remove the custom headers from buildHeaders() in dist/client.js:

```js
buildHeaders(token, options) {
const headers = {
Authorization: `Bearer ${token}`,
};
return headers;
}
```

Environment

- mcp-aiven: 0.1.5
- Transport: stdio
- Node: v20.20.2
- OS: Linux