fix(test): use /dev/null path for write-failure test on Linux

The test used /nonexistent/directory/ which root can create on Linux CI,
causing XCTAssertFalse(fileExists) to fail. /dev/null is a character
device so createDirectory always fails regardless of permissions.

Author: alexey1312

Tests/ExFigTests/Report/ExportReportIntegrationTests.swift

@@ -93,7 +93,8 @@ final class ExportReportIntegrationTests: XCTestCase {
 
     func testReportWriteFailureDoesNotThrow() {
         let ui = TerminalUI(outputMode: .quiet)
-        let invalidPath = "/nonexistent/directory/report.json"
+        // /dev/null is a character device, not a directory — createDirectory fails even as root
+        let invalidPath = "/dev/null/report.json"
 
         let report = ExportReport(
             version: ExportReport.currentVersion,