fix(everything): limit get-env to one key

[Lubrsy706]

Summary

• require a key argument for the get-env demo tool
• return only that environment variable plus a found flag instead of dumping process.env
• add tests and update docs to reflect the narrower contract

Fixes #3986.

Tests

• npm test -- __tests__/tools.test.ts
• npm run build

[Lubrsy706]

Rebased this branch onto the latest main and resolved the get-env conflict by keeping the new single-key input schema together with the current tool annotations. Local checks passed: npm run build --workspace @modelcontextprotocol/server-everything and npm test --workspace @modelcontextprotocol/server-everything -- __tests__/tools.test.ts. The refreshed PR checks are green as well.