Support middleware clients for automatic Rust trace propagation

[meoyawn]

Generated Rust clients accept a plain Reqwest client, which prevents applications from installing tracing middleware once at construction. Callers otherwise have to route each request through a separate executor.

Accept reqwest_middleware::ClientWithMiddleware and return its request builders so every generated .send() uses the configured middleware. The README now teaches one construction-time insertion point using reqwest-tracing, with compatible dependencies and the required application tracing setup.

Validation: moon run oasmith:check passed. The new Rust regression first failed because the constructor rejected the middleware client, then passed with concurrent requests built before entering their spans. It verifies distinct active traces, HTTP child spans, authentication, JSON error decoding, and streaming SSE responses.