Fix zero-argument prompt handling

[avinashgola]

Summary

• register web_search_help as a true no-argument prompt so clients may omit params.arguments
• add a regression test that exercises the SDK prompts/get handler without an arguments object

Verification

• npx vitest run tests/unit/mcp-handler.test.ts
• npm run format:check
• npm run typecheck
• npm test

Closes #358