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/Piece/CreatePieceTool.php

@@ -10,6 +10,7 @@ use App\Repository\ConstructeurRepository;
 use App\Repository\ModelTypeRepository;
 use Doctrine\ORM\EntityManagerInterface;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 use Symfony\Bundle\SecurityBundle\Security;
 
 #[McpTool(
@@ -37,7 +38,7 @@ class CreatePieceTool
         string $prix = '',
         string $modelTypeId = '',
         array $constructeurIds = [],
-    ): array {
+    ): CallToolResult {
         $this->requireRole($this->security, 'ROLE_GESTIONNAIRE');
 
         $piece = new Piece();