fix(terrain): keep known heights when a terrain request fails

A request with any point the proxy cannot answer gets 502, and the page
used to put every point of that chunk on its geoid, including the ones the
proxy had real (cached or stale) heights for. During an upstream 429 pause
that drops cached ground under every chunk that also holds a new point.

The 502 body now carries `partial`: the request's real heights in order,
null for points without one, left out when there are none. The terrain
request service attaches it to the error it throws, and the page caches
those heights and uses its geoid only for the rest. `get_terrain_height`
still reports the failure as before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/services/requests.test.mjs

@@ -94,3 +94,20 @@ test('a failed probe is retried after its backoff, not on every query', async ()
     'GET /api/overpass/status',
   ]);
 });
+
+test('a failed terrain request carries the real heights the server had', async () => {
+  const partial = [{ ellipsoid: 412.5 }, null];
+  const services = createApplicationRequestServices({
+    fetchImpl: async () => Response.json({ error: 'x', partial }, { status: 502 }),
+  });
+  const points = [{ lat: 30, lon: -97 }, { lat: 31, lon: -97 }];
+  await assert.rejects(services.terrain.getHeights(points), (error) => {
+    assert.match(error.message, /Terrain heights unavailable \(502\)/);
+    assert.deepEqual(error.partial, partial);
+    return true;
+  });
+  await assert.rejects(services.terrain.getHeights(points.slice(0, 1)), (error) => {
+    assert.equal(error.partial, undefined, 'a list of the wrong length is not attached');
+    return true;
+  });
+});