eslint configuration to permit `_`-prefix naming convention for unused vars

[richardkmichael]

• https://github.com/modelcontextprotocol/inspector/blob/main/client/src/lib/__tests__/auth.test.ts#L104-L105
• https://github.com/modelcontextprotocol/inspector/blob/f942df7e4c1f53fead4102cf14b635797d59de84/cli/src/index.ts#L93
• https://github.com/modelcontextprotocol/inspector/blob/ae7d745d5e5c5ea9979b3192272127c98fa76eb6/client/src/lib/constants.ts#L6