fix(diagnostics): emit asyncStart after promise settlement to match native tracePromise (#4824)

Previously, `asyncStart` was emitted when the promise was first returned, rather than when the promise settled. Emitting it early meant subscribers that temporarily activate context between asyncStart and asyncEnd could leak that context into overlapping sibling operations.

This PR moves asyncStart to the start of the continuing callback, matching the docs at https://nodejs.org/api/diagnostics_channel.html#asyncstartevent:

>The asyncStart event represents the callback or continuation of a traceable function being reached. At this point things like callback arguments may be available, or anything else expressing the "result" of the action.

This did not show up in earlier tests because most subscribers only used `start` for context creation and `asyncEnd` for completion. This should not be a breaking change because the terminal `end/asyncEnd` semantics and result/error payloads remain the same. This is an invisible change for most users, and is only correcting a slight mismatch.

We had to manually implement the async lifecycle in `traceMixed` here because we needed to preserve the `sync/async` return paths in resolvers.

Author: logaretm

src/diagnostics.ts

@@ -427,17 +427,18 @@ export function traceMixed<TResult, TContext extends TraceLifecycleContext>(
     }
 
     channel.end.publish(context);
-    channel.asyncStart.publish(context);
 
     return result.then(
       (value) => {
         context.result = value;
+        channel.asyncStart.publish(context);
         channel.asyncEnd.publish(context);
         return value;
       },
       (err: unknown) => {
         context.error = err;
         channel.error.publish(context);
+        channel.asyncStart.publish(context);
         channel.asyncEnd.publish(context);
         throw err;
       },

src/execution/__tests__/diagnostics-execute-test.ts

@@ -121,6 +121,7 @@ describe('execute diagnostics channel', () => {
             rawVariableValues: undefined,
             operationName: undefined,
             operationType: OperationTypeNode.QUERY,
+            result,
           },
         },
         {
@@ -201,17 +202,18 @@ describe('execute diagnostics channel', () => {
           },
         },
         {
-          channel: 'asyncStart',
+          channel: 'error',
           context: {
             schema: asyncDeferSchema,
             document,
             rawVariableValues: undefined,
             operationName: 'Deferred',
             operationType: OperationTypeNode.QUERY,
+            error,
           },
         },
         {
-          channel: 'error',
+          channel: 'asyncStart',
           context: {
             schema: asyncDeferSchema,
             document,
@@ -456,6 +458,7 @@ describe('execute root selection set diagnostics channel', () => {
             rawVariableValues: undefined,
             operationName: undefined,
             operationType: OperationTypeNode.QUERY,
+            result,
           },
         },
         {

src/execution/__tests__/diagnostics-resolve-test.ts

@@ -130,6 +130,7 @@ describe('resolve diagnostics channel', () => {
             args: {},
             isDefaultResolver: true,
             fieldPath: 'async',
+            result: 'hello-async',
           },
         },
         {
@@ -207,6 +208,7 @@ describe('resolve diagnostics channel', () => {
             args: {},
             isDefaultResolver: true,
             fieldPath: 'async',
+            result: 'hello-thenable',
           },
         },
         {
@@ -325,7 +327,7 @@ describe('resolve diagnostics channel', () => {
           },
         },
         {
-          channel: 'asyncStart',
+          channel: 'error',
           context: {
             fieldName: 'asyncFail',
             alias: 'asyncFail',
@@ -334,10 +336,11 @@ describe('resolve diagnostics channel', () => {
             args: {},
             isDefaultResolver: true,
             fieldPath: 'asyncFail',
+            error,
           },
         },
         {
-          channel: 'error',
+          channel: 'asyncStart',
           context: {
             fieldName: 'asyncFail',
             alias: 'asyncFail',

src/execution/__tests__/diagnostics-subscribe-test.ts

@@ -145,6 +145,7 @@ describe('subscribe diagnostics channel', () => {
             rawVariableValues: undefined,
             operationName: undefined,
             operationType: OperationTypeNode.SUBSCRIPTION,
+            result,
           },
         },
         {
@@ -474,6 +475,7 @@ describe('subscribe diagnostics channel', () => {
             rawVariableValues: undefined,
             operationName: 'S',
             operationType: OperationTypeNode.SUBSCRIPTION,
+            result,
           },
         },
         {