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/Site/CreateSiteTool.php

@@ -8,6 +8,7 @@ use App\Entity\Site;
 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(
@@ -31,7 +32,7 @@ class CreateSiteTool
         string $contactPostalCode = '',
         string $contactCity = '',
         string $color = '',
-    ): array {
+    ): CallToolResult {
         $this->requireRole($this->security, 'ROLE_GESTIONNAIRE');
 
         $site = new Site();