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/DashboardStatsTool.php

@@ -12,6 +12,7 @@ use App\Repository\SiteRepository;
 use Doctrine\ORM\EntityManagerInterface;
 use Mcp\Capability\Attribute\McpTool;
 use Mcp\Schema\Content\TextContent;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'get_dashboard_stats',
@@ -28,14 +29,14 @@ class DashboardStatsTool
         private readonly EntityManagerInterface $em,
     ) {}
 
-    public function __invoke(): array
+    public function __invoke(): CallToolResult
     {
         $unresolvedComments = (int) $this->em->createQuery(
             "SELECT COUNT(c.id) FROM App\\Entity\\Comment c WHERE c.status = 'open'"
         )->getSingleScalarResult();
 
-        return [
-            new TextContent(
+        return new CallToolResult(
+            content: [new TextContent(
                 text: json_encode([
                     'machines'           => $this->machines->count([]),
                     'pieces'             => $this->pieces->count([]),
@@ -44,7 +45,7 @@ class DashboardStatsTool
                     'sites'              => $this->sites->count([]),
                     'unresolvedComments' => $unresolvedComments,
                 ], JSON_THROW_ON_ERROR)
-            ),
-        ];
+            )],
+        );
     }
 }