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/Piece/GetPieceTool.php

@@ -7,6 +7,7 @@ namespace App\Mcp\Tool\Piece;
 use App\Mcp\Tool\McpToolHelper;
 use App\Repository\PieceRepository;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'get_piece',
@@ -20,7 +21,7 @@ class GetPieceTool
         private readonly PieceRepository $pieces,
     ) {}
 
-    public function __invoke(string $pieceId): array
+    public function __invoke(string $pieceId): CallToolResult
     {
         $piece = $this->pieces->find($pieceId);