fix: add next_pending_certificate on end task

[Freyskeyd]

Description

This PR adds a call to get_next_pending_certificates after a task ends in order to process next pending certificates.
It also removes some old struct/mechanism of the broadcast.

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (90405f3) 68.07% compared to head (5378b38) 68.24%.

Files	Patch %	Lines
crates/topos-tce-broadcast/src/task_manager/mod.rs	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
+ Coverage   68.07%   68.24%   +0.16%     
==========================================
  Files         223      223              
  Lines       12331    12258      -73     
==========================================
- Hits         8394     8365      -29     
+ Misses       3937     3893      -44     

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[gruberb]

Remove .unwrap()

I agree with Simon here: panics are ok in benchmarks and tests.