Have server API base URL be configurable via environment variable

[mooreds]

Is your feature request related to a problem? Please describe.

I have an OpenAPI specification that has multiple API servers. Currently I have to edit the generated src/index.ts and update the API_BASE_URL constant to switch between them.

Describe the solution you'd like

I'd like to be able to specify which API server to run my MCP server against using an environment variable.

Describe alternatives you've considered

Editing the file as part of the build process.