Add diagnostics buffer tests

tests/diagnostics.test.mjs

@@ -0,0 +1,49 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import {
+  airshipLog,
+  clearAirshipLogs,
+  getAirshipLogs,
+} from '../src/diagnostics.js';
+
+test('diagnostics records structured entries and returns copies', () => {
+  clearAirshipLogs();
+
+  const originalLog = console.log;
+  console.log = () => {};
+  try {
+    airshipLog('test:event', { value: 42 });
+  } finally {
+    console.log = originalLog;
+  }
+
+  const logs = getAirshipLogs();
+  assert.equal(logs.length, 1);
+  assert.equal(logs[0].event, 'test:event');
+  assert.equal(logs[0].value, 42);
+  assert.equal(logs[0].level, 'info');
+  assert.match(logs[0].ts, /^\d{4}-\d{2}-\d{2}T/);
+
+  logs[0].value = 99;
+  assert.equal(getAirshipLogs()[0].value, 42);
+});
+
+test('diagnostics ring buffer is capped at 300 entries', () => {
+  clearAirshipLogs();
+
+  const originalLog = console.log;
+  console.log = () => {};
+  try {
+    for (let i = 0; i < 305; i += 1) {
+      airshipLog('test:ring', { i });
+    }
+  } finally {
+    console.log = originalLog;
+  }
+
+  const logs = getAirshipLogs();
+  assert.equal(logs.length, 300);
+  assert.equal(logs[0].i, 5);
+  assert.equal(logs.at(-1).i, 304);
+});