Function getDefaultMcpServerUrl

Resolve the default MCP server URL. Priority:

1. MCP_SERVER_URL env var (explicit override — e.g. from mcp-setup.sh).
2. AWF sandbox auto-detection → gateway URL on the dynamically-resolved MCP_GATEWAY_DOMAIN:MCP_GATEWAY_PORT (gh-aw v0.69+ uses port 8080).
3. Direct onrender HTTPS endpoint (local dev / CI outside AWF sandbox).