Add Rust SDK and contract-test generators

[calvin-archastro]

Summary

• add an idiomatic Rust SDK backend with serde models, resource namespaces, async and blocking methods, auth, typed SSE, and Phoenix channel facades
• add Rust REST, SSE, and channel contract-test generation
• preserve nullable, recursive, enum, union, identifier-collision, and raw-response semantics
• expose the new rust and contract-tests-rust CLI targets

Verification

• 397 sdk-generator tests
• TypeScript build
• generated SDK compiles on Rust 1.85
• companion SDK passes 1,324 live protocol contracts
• deterministic regeneration verified

Companion SDK: ArchAstro/archastro-rust#1.