fix: Chain natively for Promise instances, side-chain for subclasses/thenables

Mirror the behaviour of Node.js core diagnostics_channel tracePromise:

- Native Promise (promise.constructor === Promise): chain via .then() and
  return the chained promise. Safe because there is no subclass API to
  preserve and the native .then() returns a native Promise.

- Promise subclass or other thenable: side-chain .then() purely for the
  asyncStart/asyncEnd event callbacks, then return the original promise
  unchanged. This preserves any subclass-specific methods on the return
  value (e.g. Anthropic SDK's APIPromise.withResponse()).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Qard

lib/transforms.js

@@ -391,9 +391,32 @@ function wrapPromise (state, node) {
         try {
           let promise = __apm$traced();
           if (typeof promise?.then !== 'function') {
-            __apm$ctx.result = promise; 
+            __apm$ctx.result = promise;
             return promise;
           }
+          // Mirror Node.js core diagnostics_channel behaviour: for native Promise
+          // instances, chain normally (safe since there is no subclass API to
+          // preserve). For Promise subclasses and other thenables, side-chain the
+          // callbacks for event publishing and return the original so that any
+          // subclass-specific methods (e.g. APIPromise.withResponse()) remain
+          // accessible to the caller.
+          if (promise instanceof Promise && promise.constructor === Promise) {
+            return promise.then(
+              result => {
+                __apm$ctx.result = result;
+                ${channelVariable}.asyncStart.publish(__apm$ctx);
+                ${channelVariable}.asyncEnd.publish(__apm$ctx);
+                return result;
+              },
+              err => {
+                __apm$ctx.error = err;
+                ${channelVariable}.error.publish(__apm$ctx);
+                ${channelVariable}.asyncStart.publish(__apm$ctx);
+                ${channelVariable}.asyncEnd.publish(__apm$ctx);
+                throw err;
+              }
+            );
+          }
           promise.then(
             result => {
               __apm$ctx.result = result;