Removed redundant onComplete function, added constructor promotion, fixed changelog

Author: Marco van Angeren

src/chat/CHANGELOG.md

@@ -18,5 +18,4 @@ CHANGELOG
    - Add `StreamableStoreInterface` which indicates `StoreInterface` implementation can be configured with streaming 
    - Add `AccumulatingStreamResult` wrapper class which adds accumulation logic & callback chaining to `StreamResult` implementations (can wrap both `Agent` and `Platform` variants) to return the full message once `Generator` is exhausted
    - Streamed responses now also create `AssistantMessage` & are added to `Store` in `Chat::submit()`
-   - Bugfixed loss of metadata in `Chat::submit()`
-
+   - Bugfixed loss of metadata in `Chat::submit()`

src/chat/src/Result/AccumulatingStreamResult.php

@@ -23,25 +23,10 @@
  */
 final class AccumulatingStreamResult
 {
-    private ?\Closure $onComplete = null;
-
     public function __construct(
         private readonly StreamResult|ToolboxStreamResult $innerResult,
-        ?\Closure $onComplete = null,
+        private ?\Closure $onComplete = null,
     ) {
-        $this->onComplete = $onComplete;
-    }
-
-    public function addOnComplete(\Closure $callback): void
-    {
-        $existingCallback = $this->onComplete;
-
-        $this->onComplete = $existingCallback
-            ? function (AssistantMessage $message) use ($existingCallback, $callback) {
-                $existingCallback($message);
-                $callback($message);
-            }
-        : $callback;
     }
 
     public function getContent(): \Generator