fix(astrolabe): resolve Psalm type errors in PDF preview code

Fix Psalm static analysis errors:
- Add return type annotations to refresh callback closures
- Use strict null comparisons instead of truthy/falsy checks
- Cast response body to string for json_decode
- Add type annotation for decoded JSON data
- Update psalm-baseline.xml to remove fixed issues

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

third_party/astrolabe/lib/Service/McpServerClient.php

@@ -646,7 +646,8 @@ class McpServerClient {
 					]
 				]
 			);
-			$data = json_decode($response->getBody(), true);
+			/** @var array{success?: bool, image?: string, page_number?: int, total_pages?: int, error?: string} $data */
+			$data = json_decode((string)$response->getBody(), true);
 
 			if (json_last_error() !== JSON_ERROR_NONE) {
 				throw new \RuntimeException('Invalid JSON response from server');