Update v16 interceptor docs for removed IOperationMessagePayload.As<T> (#9731)

Author: glen-84

website-next/content/docs/hotchocolate/migrating/migrate-from-15-to-16.md

@@ -1108,6 +1108,36 @@ We removed the following methods from the `IExecutionDiagnosticEventListener` si
 
 Some other methods also had a change in their signature - simply override them again to fix any compilation issues.
 
+## IOperationMessagePayload exposes raw JSON
+
+The `IOperationMessagePayload` interface, used by `ISocketSessionInterceptor` hooks (`OnConnectAsync`, `OnPingAsync`, `OnPongAsync`), no longer exposes the `As<T>()` deserialization helper. It now provides direct access to the raw `JsonElement?` through a `Payload` property:
+
+```diff
+-public interface IOperationMessagePayload
+-{
+-    T? As<T>() where T : class;
+-}
++public interface IOperationMessagePayload
++{
++    JsonElement? Payload { get; }
++}
+```
+
+If you were calling `.As<T>()` to deserialize the payload, switch to `Payload?.Deserialize<T>()`:
+
+```diff
+public override ValueTask<ConnectionStatus> OnConnectAsync(
+    ISocketSession session,
+    IOperationMessagePayload connectionInitMessage,
+    CancellationToken cancellationToken = default)
+{
+-   var payload = connectionInitMessage.As<MyConnectPayload>();
++   var payload = connectionInitMessage.Payload?.Deserialize<MyConnectPayload>();
+
+    // ...
+}
+```
+
 ## Experimental @semanticNonNull support removed
 
 Hot Chocolate v15 included experimental support for the `@semanticNonNull` directive, which let you mark fields as semantically non-null while still returning `null` (rather than propagating to the parent) when a resolver errored. We've removed this feature in v16 in favor of the [`onError` proposal](https://github.com/graphql/graphql-spec/pull/1163).

website/src/docs/hotchocolate/v16/server/interceptors.md

@@ -176,9 +176,11 @@ public override ValueTask<ConnectionStatus> OnConnectAsync(
     ISocketSession session, IOperationMessagePayload connectionInitMessage,
     CancellationToken cancellationToken)
 {
-    if (connectionInitMessage.As<Dictionary<string, object?>>()
-        ?.TryGetValue("authToken", out var token) == true)
+    if (connectionInitMessage.Payload is { ValueKind: JsonValueKind.Object } payload
+        && payload.TryGetProperty("authToken", out var token)
+        && token.ValueKind == JsonValueKind.String)
     {
+        var authToken = token.GetString();
         // Validate token ...
     }