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/CustomField/UpsertCustomFieldValuesTool.php

@@ -13,6 +13,7 @@ use App\Entity\Product;
 use App\Mcp\Tool\McpToolHelper;
 use Doctrine\ORM\EntityManagerInterface;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 use Symfony\Bundle\SecurityBundle\Security;
 
 #[McpTool(
@@ -30,7 +31,7 @@ class UpsertCustomFieldValuesTool
         private readonly Security $security,
     ) {}
 
-    public function __invoke(string $entityType, string $entityId, array $fields): array
+    public function __invoke(string $entityType, string $entityId, array $fields): CallToolResult
     {
         $this->requireRole($this->security, 'ROLE_GESTIONNAIRE');