test(images): bound the shared-listing test waits (#348)

A macOS py3.12 run stopped inside the shared-listing tests and sat until
the 6 h job limit with no traceback. It does not reproduce locally (about
300 runs on 3.12 and 3.14, pinned to one core). Requests and background
settles in TestSharedListing now give up after 30 s, so a recurrence
fails with the stuck await instead of hanging the run.

tests/test_api_images.py

@@ -2351,6 +2351,10 @@ class TestRootScopeServing(ImageAPITestCase):
                 await self.assert_no_access(path, scope=token)
 
 
+# Seconds any single wait in the shared-listing tests may take.
+_HANG_LIMIT = 30
+
+
 class _GatedScan:
     """Wraps the real scan, counts calls and can hold one call after it listed.
 
@@ -2404,8 +2408,13 @@ class TestSharedListing(ImageAPITestCase):
         self.addCleanup(self.scan.release_all)
 
     async def _json(self, method, path, **kwargs):
-        resp = await self.client.request(method, path, **kwargs)
-        return resp.status, await resp.json()
+        # Bounded, like every wait in this class: a stuck walk fails the test
+        # with a traceback instead of hanging the whole run.
+        async def call():
+            resp = await self.client.request(method, path, **kwargs)
+            return resp.status, await resp.json()
+
+        return await asyncio.wait_for(call(), _HANG_LIMIT)
 
     def _record(self):
         return self.api._root_listings[str(self.output_dir)]
@@ -2418,8 +2427,12 @@ class TestSharedListing(ImageAPITestCase):
 
     async def _settle(self):
         """Wait for every background walk this API started."""
-        while self.api._listing_tasks:
-            await asyncio.gather(*list(self.api._listing_tasks))
+
+        async def settle():
+            while self.api._listing_tasks:
+                await asyncio.gather(*list(self.api._listing_tasks))
+
+        await asyncio.wait_for(settle(), _HANG_LIMIT)
 
     async def test_concurrent_cold_readers_share_one_walk(self):
         make_png(self.output_dir / "sub" / "a.png")