Suite validation accepts empty prompts and bogus message roles

[royalpinto007]

Problem

A suite with "prompt": "", "messages": [], or {role: "bogus"} passes validation and fails confusingly downstream.

Where

src/suite.ts (validateSuite): only checks typeof input.prompt === "string" and Array.isArray(input.messages).

Acceptance

Add a failing test first asserting validateSuite rejects empty-string prompts, empty messages arrays, and unknown message roles. Then fix.