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/Document/ListDocumentsTool.php

@@ -7,6 +7,7 @@ namespace App\Mcp\Tool\Document;
 use App\Mcp\Tool\McpToolHelper;
 use App\Repository\DocumentRepository;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'list_documents',
@@ -28,7 +29,7 @@ class ListDocumentsTool
         private readonly DocumentRepository $documents,
     ) {}
 
-    public function __invoke(string $entityType, string $entityId): array
+    public function __invoke(string $entityType, string $entityId): CallToolResult
     {
         if (!isset(self::ENTITY_FIELDS[$entityType])) {
             $this->mcpError('validation', "Invalid entityType '{$entityType}'. Must be one of: site, machine, composant, piece, product.");