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/Comment/UnresolvedCountTool.php

@@ -8,6 +8,7 @@ use App\Entity\Comment;
 use App\Mcp\Tool\McpToolHelper;
 use Doctrine\ORM\EntityManagerInterface;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'get_unresolved_comments_count',
@@ -21,7 +22,7 @@ class UnresolvedCountTool
         private readonly EntityManagerInterface $em,
     ) {}
 
-    public function __invoke(): array
+    public function __invoke(): CallToolResult
     {
         $count = (int) $this->em->getRepository(Comment::class)
             ->createQueryBuilder('c')