feat(truapi): add testing API and versioned wiring

[pgherveou]

Adds the canonical testing module (api/testing.rs) and its v01/v02/versioned wiring used by the Rust host runtime and generated clients.

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Stack

Part of the Rust core runtime port (umbrella: #104), split into a reviewable stack. Merge bottom-up.

1. feat(truapi): add testing API and versioned wiring #248 — truapi: testing API + versioned wiring 👈
2. feat(truapi-platform): add host capability traits #249 — truapi-platform: host capability traits
3. feat(truapi-server): add Rust host runtime #250 — truapi-server: Rust host runtime
4. feat(truapi-codegen): emit Rust dispatcher, wire table, and host callbacks #251 — truapi-codegen: Rust dispatcher + wire table
5. feat(host-wasm): add @parity/truapi-host-wasm runtime #252 — host-wasm: @parity/truapi-host-wasm runtime
6. chore: docs, CI, tooling, and dotli integration for the Rust core #253 — docs, CI, tooling, dotli integration