test: reproduce stale browser validation result (#4)

tests/launch-failure.cjs

@@ -0,0 +1,20 @@
+// Reproduce an unavailable-browser failure without requiring a browser/server.
+const assert=require('node:assert/strict');
+const fs=require('node:fs');
+const path=require('node:path');
+const {spawnSync}=require('node:child_process');
+const folder=path.resolve(__dirname,'../test-results/launch-failure-case');
+fs.mkdirSync(folder,{recursive:true});
+const result=path.join(folder,'ui-validation.json');
+fs.writeFileSync(result,JSON.stringify({status:'passed',previous_run:true}));
+const run=spawnSync(process.execPath,[path.join(__dirname,'viewer.cjs')],{
+ env:{...process.env,PLAYWRIGHT_CHANNEL:'intentionally-invalid-channel',VIEWER_RESULTS_DIR:folder},
+ encoding:'utf8',timeout:20000
+});
+if(run.error)throw run.error;
+assert.equal(run.status,1,run.stderr||run.stdout);
+const report=JSON.parse(fs.readFileSync(result,'utf8'));
+assert.equal(report.status,'failed');
+assert.match(report.error,/intentionally-invalid-channel/);
+assert.equal(report.previous_run,undefined);
+console.log('PASS: browser launch failure returns 1 and overwrites stale success JSON.');