chore: disable chromium sandbox in tests on all CI platforms

The 2026-07-08 Playwright roll enables the chromium sandbox by default
for the default MCP session; pass --no-sandbox on all CI platforms
(previously Linux-only) so Windows CI does not hang on browser launch.

Author: yury-s

tests/fixtures.ts

@@ -80,7 +80,7 @@ export const test = baseTest.extend<TestFixtures & TestOptions, WorkerFixtures>(
     await use(async options => {
       const cwd = testInfo.outputPath();
       const args: string[] = mcpArgs ?? [];
-      if (process.env.CI && process.platform === 'linux')
+      if (process.env.CI)
         args.push('--no-sandbox');
       if (mcpHeadless)
         args.push('--headless');