Bound the Dell warranty sync per run

The sync route asked Dell about every pending service tag in one request.
bulk_lookup sends 100 tags per HTTP call at a 15 second timeout, so a
freshly-onboarded site was dozens of sequential calls inside a single web
request. A gunicorn worker timeout or a proxy would plausibly kill it first,
leaving the user with a failure after minutes and no way to tell what landed.
F3 released the pooled connection before the vendor call, which stopped one
sync from taking the app down, but did not make the request itself survivable.

Extract the sync into a service taking a per-run tag limit. The route now caps
at 200 tags - two Dell calls - and returns how many are still pending so the
caller can run it again. A flask warranty sync-dell command does the same work
from the command line for a large first-run backfill.

The chunk is sampled at random rather than taken off a stable order. A serial
Dell does not recognize never gains a warranty and so stays pending forever;
under a stable order those dead tags would occupy every chunk and the sync
would never reach the rest of the fleet.

Nothing runs this on a schedule. The operation is used a few times a year, so
a cron would idle-poll the warranty tables tens of thousands of times a year
for nothing and log a configuration failure on every run at sites without Dell
credentials.

The two existing sync tests patched get_provider on the route module; the
provider is now resolved in the service, so they patch it there.

tests/test_plugins/test_warranty_connection_release.py

@@ -14,6 +14,7 @@ continues to pass.
 """
 
 from plugins.warranty.api import routes as warranty_routes
+from plugins.warranty.services import dellsync as warranty_dellsync
 from plugins.warranty.models import Warranty, WarrantyAsset
 
 
@@ -104,7 +105,7 @@ def test_sync_dell_releases_the_connection_before_the_bulk_call(
         'startdate': '2023-05-02',
         'enddate': '2026-05-02',
     })
-    monkeypatch.setattr(warranty_routes, 'get_provider', lambda name: probe)
+    monkeypatch.setattr(warranty_dellsync, 'get_provider', lambda name: probe)
 
     response = client.post('/api/warranty/sync/dell', headers=auth_headers)
 
@@ -147,7 +148,7 @@ def test_sync_dell_updates_instead_of_duplicating_on_a_recheck(
         'startdate': '2024-02-02',
         'enddate': '2028-02-02',
     })
-    monkeypatch.setattr(warranty_routes, 'get_provider', lambda name: probe)
+    monkeypatch.setattr(warranty_dellsync, 'get_provider', lambda name: probe)
 
     response = client.post('/api/warranty/sync/dell?all=true', headers=auth_headers)