Mock provider: request-shape strictness — non-integer/string ids and missing params #79

Audit findings L1 + L2 — verified live.

Problem

• server.rs:127-139 accepts any JSON Number as a request id — "id": 1.5 is accepted and echoed though spec §4 requires integer or string — and maps "id": true / [1] / {} to notificationNotSupported even though those messages have an id (spec-literal answer: -32600 Invalid Request).
• server.rs:140 defaults a missing params to {} — {"method":"lpp/shutdown"} without params succeeds though §4 marks params REQUIRED.

Acceptance criteria

• Non-integer/non-string ids (including floats) -> -32600; absent/null id keeps the pinned notificationNotSupported path.
• Requests missing params are rejected consistently with the schema (-32602).
• Regression tests + green suite.