test: robust fixes for windows hook flakiness

- Enforce 'sequential: true' for all hook tests to prevent telemetry leaks and race conditions.
- Normalize all path assertions in hooks-system.test.ts using a new 'normalizePath' helper to handle Windows backslashes consistently.
- Update 'createScript' in test-rig to return normalized paths.
- Ensure 'PATH' is explicitly passed to node-pty spawn options to prevent 'posix_spawnp' errors in some environments.
- Clean up manual path replacements in tests in favor of the centralized helper.

Part of https://github.com/google-gemini/gemini-cli/pull/18665

packages/test-utils/src/test-rig.ts

@@ -60,6 +60,14 @@ export function sanitizeTestName(name: string) {
     .replace(/-+/g, '-');
 }
 
+/**
+ * Normalizes a path for use in command-line arguments.
+ * On Windows, this converts backslashes to forward slashes.
+ */
+export function normalizePath(p: string): string {
+  return p.replace(/\\/g, '/');
+}
+
 // Helper to create detailed error messages
 export function createToolCallErrorMessage(
   expectedTools: string | string[],
@@ -446,7 +454,7 @@ export class TestRig {
     }
     const scriptPath = join(this.testDir, fileName);
     writeFileSync(scriptPath, content);
-    return scriptPath;
+    return normalizePath(scriptPath);
   }
 
   mkdir(dir: string) {
@@ -1295,6 +1303,11 @@ export class TestRig {
 
     const envVars = this._getCleanEnv(options?.env);
 
+    // Ensure PATH is included for pty
+    if (!envVars['PATH'] && process.env['PATH']) {
+      envVars['PATH'] = process.env['PATH'];
+    }
+
     const ptyOptions: pty.IPtyForkOptions = {
       name: 'xterm-color',
       cols: 80,