Fall back to next free port when preferred port is busy in development

[hammadsaj]

Summary

• In development/debug, if CORTEX_PORT is already in use, bind the next available port instead of failing with EADDRINUSE
• Log a warning when falling back and update config/CORTEX_PORT to the bound port
• Production still fails hard if the preferred port is busy

Test plan

• With nothing on 4000, npm start / npm run dev binds to 4000 as usual
• With another process on 4000, the server binds to the next free port and logs the fallback warning
• Confirm GraphQL playground opens at the logged URL
• npm test -- tests/unit/server/listenHttpServer.test.js

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