fix(mcp) : return CallToolResult to prevent structuredContent serialization issue

Tools now return CallToolResult directly instead of Content arrays,
preventing the MCP SDK from auto-generating structuredContent as a
JSON array (which Claude Code rejects — expects a JSON object/record).
Also adds Accept header to test helpers and SSE response parsing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Mcp/Tool/McpToolHelper.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\Mcp\Tool;
 
 use Mcp\Schema\Content\TextContent;
+use Mcp\Schema\Result\CallToolResult;
 use RuntimeException;
 use Symfony\Bundle\SecurityBundle\Security;
 
@@ -17,12 +18,11 @@ trait McpToolHelper
         }
     }
 
-    /**
-     * @return array{TextContent}
-     */
-    private function jsonResponse(array $data): array
+    private function jsonResponse(array $data): CallToolResult
     {
-        return [new TextContent(text: json_encode($data, JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE))];
+        return new CallToolResult(
+            content: [new TextContent(text: json_encode($data, JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE))],
+        );
     }
 
     private function mcpError(string $category, string $message): never
@@ -42,10 +42,7 @@ trait McpToolHelper
         return ['page' => $page, 'limit' => $limit, 'offset' => $offset];
     }
 
-    /**
-     * @return array{TextContent}
-     */
-    private function paginatedResponse(array $items, int $total, int $page, int $limit): array
+    private function paginatedResponse(array $items, int $total, int $page, int $limit): CallToolResult
     {
         return $this->jsonResponse([
             'items'     => $items,