Document OpenTelemetry trace propagation

[meoyawn]

What changed

• document OpenTelemetry trace propagation for generated Go clients through context.Context and otelhttp.Transport
• document propagation for generated TypeScript clients through the injectable fetch hook
• state the required SDK, active span context, context manager, and text-map propagator prerequisites

Why

Generated clients expose the necessary transport hooks, but users need concrete setup examples to preserve distributed traces across API calls. The TypeScript guidance also makes clear that @opentelemetry/api alone uses no-op propagation and does not emit traceparent.

Validation

• git diff --check
• runtime propagation proof with @opentelemetry/api@1.9.1 and W3CTraceContextPropagator: no registered propagator produced no traceparent; a valid active span plus the W3C propagator produced the expected header