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/MachineStructureTool.php

@@ -21,6 +21,7 @@ use App\Repository\MachineProductLinkRepository;
 use App\Repository\MachineRepository;
 use Doctrine\Common\Collections\Collection;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'get_machine_structure',
@@ -37,7 +38,7 @@ class MachineStructureTool
         private readonly MachineProductLinkRepository $machineProductLinkRepository,
     ) {}
 
-    public function __invoke(string $machineId): array
+    public function __invoke(string $machineId): CallToolResult
     {
         $machine = $this->machineRepository->find($machineId);