minor #763 [Agent] Drop mixed in favor of ToolResult in event (chr-hertel)

This PR was merged into the main branch.

Discussion
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[Agent] Drop mixed in favor of ToolResult in event

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Docs?         | no
| Issues        |
| License       | MIT

Found another `mixed` that is not needed and just dangerous

Commits
-------

5da2a28 Drop mixed in favor of ToolResult in event

Author: chr-hertel

src/agent/src/Toolbox/Event/ToolCallSucceeded.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\AI\Agent\Toolbox\Event;
 
+use Symfony\AI\Agent\Toolbox\ToolResult;
 use Symfony\AI\Platform\Tool\Tool;
 
 /**
@@ -25,7 +26,7 @@ public function __construct(
         private object $tool,
         private Tool $metadata,
         private array $arguments,
-        private mixed $result,
+        private ToolResult $result,
     ) {
     }
 
@@ -47,7 +48,7 @@ public function getArguments(): array
         return $this->arguments;
     }
 
-    public function getResult(): mixed
+    public function getResult(): ToolResult
     {
         return $this->result;
     }

src/agent/src/Toolbox/Toolbox.php

@@ -82,7 +82,7 @@ public function execute(ToolCall $toolCall): ToolResult
 
             $arguments = $this->argumentResolver->resolveArguments($metadata, $toolCall);
             $this->eventDispatcher?->dispatch(new ToolCallArgumentsResolved($tool, $metadata, $arguments));
-            $result = $tool->{$metadata->getReference()->getMethod()}(...$arguments);
+            $result = new ToolResult($toolCall, $tool->{$metadata->getReference()->getMethod()}(...$arguments));
             $this->eventDispatcher?->dispatch(new ToolCallSucceeded($tool, $metadata, $arguments, $result));
         } catch (ToolExecutionExceptionInterface $e) {
             $this->eventDispatcher?->dispatch(new ToolCallFailed($tool, $metadata, $arguments ?? [], $e));
@@ -93,7 +93,7 @@ public function execute(ToolCall $toolCall): ToolResult
             throw ToolExecutionException::executionFailed($toolCall, $e);
         }
 
-        return new ToolResult($toolCall, $result);
+        return $result;
     }
 
     private function getMetadata(ToolCall $toolCall): Tool