feat: load Google 3D with short-lived server tokens

Without a browser key, the app asks its server for a short-lived Google
access token (GET /api/google/tiles-token, answering accessToken and
expiresAt) and loads Photorealistic 3D Tiles directly with it as a
header. A tile refused for an expired token gets a renewed one and is
retried once. A server that does not offer tokens changes nothing: the
app continues through ion or the keyless globe as before.

src/maps/googleTokens.test.mjs

@@ -0,0 +1,64 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { createGoogleTokenSource, GOOGLE_TOKEN_PATH } from './googleTokens.js';
+
+function server(answers) {
+  const calls = [];
+  return {
+    calls,
+    fetchImpl: async (url) => {
+      calls.push(url);
+      const answer = answers.shift();
+      if (answer instanceof Error) throw answer;
+      return answer;
+    },
+  };
+}
+
+const offer = (accessToken, expiresAt) =>
+  Response.json({ accessToken, expiresAt });
+
+test('a token is reused until shortly before it expires', async () => {
+  let now = 1_000_000;
+  const { calls, fetchImpl } = server([
+    offer('first', now + 900_000),
+    offer('second', now + 1_800_000),
+  ]);
+  const tokens = createGoogleTokenSource({ fetchImpl, now: () => now });
+  assert.equal(await tokens.token(), 'first');
+  assert.equal(await tokens.token(), 'first');
+  assert.deepEqual(calls, [GOOGLE_TOKEN_PATH]);
+  now += 850_000;
+  assert.equal(await tokens.token(), 'second');
+  assert.equal(calls.length, 2);
+});
+
+test('requests refused for the same token renew it once', async () => {
+  const now = 1_000_000;
+  const { calls, fetchImpl } = server([
+    offer('first', now + 900_000),
+    offer('second', now + 900_000),
+  ]);
+  const tokens = createGoogleTokenSource({ fetchImpl, now: () => now });
+  await tokens.token();
+  const renewed = await Promise.all([
+    tokens.token({ replacing: 'first' }),
+    tokens.token({ replacing: 'first' }),
+  ]);
+  assert.deepEqual(renewed, ['second', 'second']);
+  assert.equal(await tokens.token({ replacing: 'first' }), 'second');
+  assert.equal(calls.length, 2);
+});
+
+test('a server that does not offer tokens yields none', async () => {
+  for (const answer of [
+    new Response('Not found', { status: 404 }),
+    new Response('<!doctype html>', { status: 200 }),
+    Response.json({ accessToken: '', expiresAt: Date.now() + 60_000 }),
+    Response.json({ accessToken: 'stale', expiresAt: 1 }),
+    new TypeError('offline'),
+  ]) {
+    const { fetchImpl } = server([answer]);
+    assert.equal(await createGoogleTokenSource({ fetchImpl }).token(), null);
+  }
+});