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/ListMachineLinksTool.php

@@ -10,6 +10,7 @@ use App\Repository\MachinePieceLinkRepository;
 use App\Repository\MachineProductLinkRepository;
 use App\Repository\MachineRepository;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'list_machine_links',
@@ -26,7 +27,7 @@ class ListMachineLinksTool
         private readonly MachineProductLinkRepository $productLinks,
     ) {}
 
-    public function __invoke(string $machineId): array
+    public function __invoke(string $machineId): CallToolResult
     {
         $machine = $this->machines->find($machineId);
         if (null === $machine) {