feat: Harden input validation across models and expand test coverage

[yukik8]

Builds on #10. This issue assumes the models are realigned to the official schema and covers only what #10 does not: enforcing invariants at input boundaries, and the test coverage to keep them enforced.

Is your feature request related to a problem?

Invalid input is not rejected at the boundary where it enters the SDK. It either travels to the API and fails there, or — worse — degrades silently. With the official schema as the baseline:

• Client entry point: evaluate() / evaluateAsync() accept an empty question list or a null state and send the request anyway.
• Duplicate question names: questions are keyed by name in the request (questions map), so two primitives sharing a name silently collapse into one — no error, one answer just disappears.
• Model invariants: post-bug: Native request/response format doesn't match the official API schema #10, Choice.criteria is a map of option name → description. Blank option names and an empty map should be rejected at construction, like Score already rejects invalid levels (feat: Support Vercel AI Gateway as an alternative transport #7) and Choice already caps options at 255.

Proposed Solution / Feature

• evaluate() / evaluateAsync(): throw JevValidationException on a null state, an empty primitives list, or duplicate primitive names. One check at the entry point guards both the native and gateway transports.
• Choice: reject an empty criteria map and blank option names in the compact constructor.
• Tests for every invariant above, plus the existing ones (Score level bounds, Noul/Choice/Score name validation, builder API-key check) — currently only two tests exist outside the gateway suite.

Example API Usage

jev.evaluate(null, questions);                          // -> JevValidationException (no state)
jev.evaluate("state", List.of());                       // -> JevValidationException (no questions)
jev.evaluate("state", List.of(
        new Noul("a", "?"), new Choice("a", "?", ...))); // -> JevValidationException (duplicate name "a")

Additional Context

Happy to implement together with the tests once #10 lands.