refactor: add cross-platform utility functions

server/src/shared/platform/shell.test.ts

@@ -0,0 +1,43 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { buildFallbackCommand, createShellSpawnPlan, quoteShellArgument } from './shell.js';
+
+// This test verifies the backend can ask for a shell launch without branching on the OS at every call site.
+test('createShellSpawnPlan returns the expected executable and argv per platform', () => {
+  assert.deepEqual(createShellSpawnPlan('echo hello', 'windows'), {
+    platform: 'windows',
+    executable: 'powershell.exe',
+    args: ['-Command', 'echo hello'],
+    commandFlag: '-Command',
+    preferredLineEnding: 'crlf',
+    pathSeparator: '\\',
+  });
+
+  assert.deepEqual(createShellSpawnPlan('echo hello', 'linux'), {
+    platform: 'linux',
+    executable: 'bash',
+    args: ['-c', 'echo hello'],
+    commandFlag: '-c',
+    preferredLineEnding: 'lf',
+    pathSeparator: '/',
+  });
+});
+
+// This test verifies shell quoting rules stay isolated inside the adapter layer.
+test('quoteShellArgument escapes embedded single quotes correctly', () => {
+  assert.equal(quoteShellArgument("it's", 'windows'), "'it''s'");
+  assert.equal(quoteShellArgument("it's", 'linux'), `'it'"'"'s'`);
+});
+
+// This test verifies resume-or-fallback command composition stays platform-specific in one helper.
+test('buildFallbackCommand emits PowerShell or POSIX fallback syntax', () => {
+  assert.equal(
+    buildFallbackCommand("codex resume '123'", 'codex', 'windows'),
+    "codex resume '123'; if ($LASTEXITCODE -ne 0) { codex }",
+  );
+  assert.equal(
+    buildFallbackCommand("codex resume '123'", 'codex', 'linux'),
+    "codex resume '123' || codex",
+  );
+});