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/Machine/ListMachinesTool.php

@@ -7,6 +7,7 @@ namespace App\Mcp\Tool\Machine;
 use App\Mcp\Tool\McpToolHelper;
 use App\Repository\MachineRepository;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'list_machines',
@@ -20,7 +21,7 @@ class ListMachinesTool
         private readonly MachineRepository $machines,
     ) {}
 
-    public function __invoke(int $page = 1, int $limit = 30, string $search = ''): array
+    public function __invoke(int $page = 1, int $limit = 30, string $search = ''): CallToolResult
     {
         $p = $this->paginationParams($page, $limit);