fix: remove b-prefix in background task events function

[lizable]

This PR removes b-prefix in sending server_closed due to innate code in "aiohttp-sse".

[codecov]

Codecov Report

Merging #394 (2222c42) into master (02ebb0d) will decrease coverage by 1.98%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
- Coverage   48.56%   46.58%   -1.99%     
==========================================
  Files          51       49       -2     
  Lines        7898     7841      -57     
==========================================
- Hits         3836     3653     -183     
- Misses       4062     4188     +126     

Impacted Files	Coverage Δ
src/ai/backend/gateway/events.py	50.52% <0.00%> (ø)
src/ai/backend/gateway/ratelimit.py	45.61% <0.00%> (-47.37%)	⬇️
src/ai/backend/manager/plugin/__init__.py	50.00% <0.00%> (-23.26%)	⬇️
src/ai/backend/gateway/auth.py	37.71% <0.00%> (-15.15%)	⬇️
src/ai/backend/gateway/utils.py	50.25% <0.00%> (-8.63%)	⬇️
src/ai/backend/gateway/server.py	49.18% <0.00%> (-7.66%)	⬇️
src/ai/backend/manager/models/base.py	47.86% <0.00%> (-2.57%)	⬇️
src/ai/backend/manager/plugin/exceptions.py
src/ai/backend/manager/plugin/error_monitor.py

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