fix(terrain): end retries early on a long Retry-After; cover the 429 pause

A Retry-After longer than the 10 s retry budget used to sleep the whole
budget out and then give up, so the proxy's 429 pause started 10 s late
and every new-point poll in that window still went upstream and logged.
The retry loop now stops at once in that case.

Tests: the retry loop no longer sleeps for an unreachable Retry-After;
fully cached points, including stale ones, are served during the pause
with no upstream call; a 503 does not pause; a shorter concurrent
Retry-After never cuts the pause short. CURRENT-STATE now says which
requests are still served and which log.

src/tooling/environmentProviders.test.mjs

@@ -476,3 +476,91 @@ test('a far-off Retry-After pauses the terrain upstream for at most 10 minutes',
   t.mock.timers.setTime(pausedAt + 10 * 60_000 + 1);
   assert.equal((await request('/?points=3,4')).status, 200, 'pause over');
 });
+
+/** Drive mocked timers until `promise` settles; the retry budget is 10 s. */
+async function settleOnMockedTime(t, promise) {
+  let settled = false;
+  const tracked = promise.finally(() => {
+    settled = true;
+  });
+  for (let step = 0; step < 60 && !settled; step += 1) {
+    await new Promise((resolve) => setImmediate(resolve));
+    t.mock.timers.tick(1_000);
+  }
+  assert.ok(settled, 'the request settles within its retry budget');
+  return tracked;
+}
+
+test('during a terrain 429 pause fully cached points, even stale ones, are still served', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  let limited = false;
+  t.mock.method(globalThis, 'fetch', async () => {
+    calls += 1;
+    return limited
+      ? new Response('', { status: 429, headers: { 'retry-after': '300' } })
+      : Response.json({ results: [{ ellipsoid: 100 }] });
+  });
+  const request = install(terrainHeightsProxy());
+  assert.equal((await request('/?points=5,6')).status, 200);
+  // Past the 30-day TTL, so 5,6 is stale and would normally be refreshed.
+  t.mock.timers.setTime(Date.now() + 31 * 24 * 3600_000);
+  limited = true;
+  const first = await settleOnMockedTime(t, request('/?points=1,2'));
+  assert.equal(first.status, 502);
+  assert.equal(calls, 2, 'a Retry-After past the budget is not retried');
+  const stale = await request('/?points=5,6');
+  assert.equal(stale.status, 200);
+  assert.deepEqual(json(stale).results, [{ ellipsoid: 100 }]);
+  assert.equal((await request('/?points=5,6;7,8')).status, 502);
+  assert.equal(calls, 2, 'no upstream call during the pause');
+});
+
+test('a terrain upstream failure other than 429 does not pause later requests', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  t.mock.method(globalThis, 'fetch', async () => {
+    calls += 1;
+    return new Response('', { status: 503 });
+  });
+  const request = install(terrainHeightsProxy());
+  const first = await settleOnMockedTime(t, request('/?points=1,2'));
+  assert.equal(first.status, 502);
+  const afterFirst = calls;
+  const second = await settleOnMockedTime(t, request('/?points=3,4'));
+  assert.equal(second.status, 502);
+  assert.ok(calls > afterFirst, 'new points still go upstream after a 503');
+});
+
+test('a shorter concurrent Retry-After never cuts a terrain 429 pause short', async (t) => {
+  isolate(t);
+  t.mock.timers.enable({ apis: ['Date', 'setTimeout'], now: 1_000_000 });
+  let calls = 0;
+  t.mock.method(globalThis, 'fetch', async (raw) => {
+    calls += 1;
+    const points = new URL(raw).searchParams.get('points');
+    if (points.startsWith('0.'))
+      return Response.json({ results: [{ ellipsoid: 1 }] });
+    const retryAfter = points.startsWith('1.') ? '300' : '30';
+    return new Response('', {
+      status: 429,
+      headers: { 'retry-after': retryAfter },
+    });
+  });
+  const request = install(terrainHeightsProxy());
+  // Load the disk cache first so both requests below reach the upstream in
+  // order: the 300 s answer lands first, the 30 s answer after it.
+  assert.equal((await request('/?points=0,0')).status, 200);
+  const [long, short] = await settleOnMockedTime(
+    t,
+    Promise.all([request('/?points=1,2'), request('/?points=3,4')]),
+  );
+  assert.equal(long.status, 502);
+  assert.equal(short.status, 502);
+  const afterBoth = calls;
+  t.mock.timers.setTime(Date.now() + 120_000);
+  assert.equal((await request('/?points=5,6')).status, 502);
+  assert.equal(calls, afterBoth, 'still paused by the 300 s Retry-After');
+});