refactor(instrumentation): Separate API wrapping from tracing

[Luca Forstner (lforst)]

• API wrapping previously carried tracing lifecycle machinery. intercept and invoke now only wrap function calls, so they can be reused without creating spans or initializing Braintrust.
• Rename the internal definition API to defineInterceptor. Interceptors receive the original function, receiver, and arguments:

  const hooks = defineInterceptor("client", {
    request: channel<[string], Promise<string>>({ channelName: "request" }),
  });
  
  // Wrap calls without enabling tracing.
  hooks.request.intercept((target, receiver, args) => {
    console.log("Request:", args[0]);
    return Reflect.apply(target, receiver, args);
  });

• Move span creation, context propagation, and completion into separate tracing functions. Provider integrations explicitly register these through interceptors; manual and generated wrappers only invoke hooks.
• Remove the old tracing hook protocol. Applications with previously transformed bundles need to rebuild after upgrading.