fix(neon_framework): Handle delete-multiple background push notifications

[provokateurin]

I discovered this while investigating #2591 (I even first thought this was the initial problem).
Added documentation upstream (nextcloud/notifications#2089) because it was not documented back when it was added (nextcloud/notifications#1429).

[codecov]

Codecov Report

Attention: Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.

Project coverage is 28.85%. Comparing base (043e219) to head (86c401f).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/api/notifications/notifications_helpers.g.dart	64.86%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2603      +/-   ##
==========================================
+ Coverage   28.84%   28.85%   +0.01%     
==========================================
  Files         370      370              
  Lines      136488   136526      +38     
==========================================
+ Hits        39369    39394      +25     
- Misses      97119    97132      +13     

Flag	Coverage Δ		*Carryforward flag
account_repository	98.47% <ø> (ø)
cookie_store	99.48% <ø> (ø)		Carriedforward from 043e219
dashboard_app	96.05% <ø> (ø)
dynamite	31.05% <ø> (ø)		Carriedforward from 043e219
dynamite_end_to_end_test	61.69% <ø> (ø)		Carriedforward from 043e219
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from 043e219
interceptor_http_client	97.18% <ø> (ø)		Carriedforward from 043e219
neon_dashboard	96.05% <ø> (ø)		Carriedforward from 043e219
neon_framework	61.15% <100.00%> (+0.04%)	⬆️
neon_http_client	97.50% <ø> (ø)
neon_notifications	100.00% <ø> (ø)		Carriedforward from 043e219
neon_storage	94.66% <ø> (ø)
neon_talk	99.45% <ø> (ø)		Carriedforward from 043e219
nextcloud	24.33% <65.78%> (+0.01%)	⬆️
notifications_app	97.40% <ø> (ø)
notifications_push_repository	98.11% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from 043e219
talk_app	98.94% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
...kages/neon_framework/lib/src/utils/push_utils.dart	97.39% <100.00%> (+0.09%)	⬆️
...b/src/api/notifications/notifications_helpers.dart	83.33% <100.00%> (+1.51%)	⬆️
...src/api/notifications/notifications_helpers.g.dart	85.43% <64.86%> (-6.95%)	⬇️

[Leptopoda]

LGTM