fix(nexus): fix potential race in config debouncer

[dmitryduev]

Fixes

• Fixes WB-NNNNN
• Fixes #NNNN

Description

What does the PR do?

🤖 Generated by Copilot at e0409bc

Refactored and improved the debouncing logic for config and metric updates in the nexus server and client. Simplified the debounce package and its tests, and updated the system tests to match the new behavior.

Testing

How was this PR tested?

Checklist

• Include reference to internal ticket "Fixes WB-NNNNN" and/or GitHub issue "Fixes #NNNN" (if applicable)
• Ensure PR title compliance with the conventional commits standards

🤖 Generated by Copilot at e0409bc

We've got a new debouncer in our code, me hearties
It's simpler and faster than the old one, so they say
So let's test it well and make sure it works, on the count of three
Heave ho, heave ho, configDebouncer away

[codecov]

Codecov Report

Merging #6385 (af75ae2) into main (50d613f) will decrease coverage by 3.87%.
Report is 1 commits behind head on main.
The diff coverage is 85.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6385      +/-   ##
==========================================
- Coverage   81.03%   77.16%   -3.87%     
==========================================
  Files         362      388      +26     
  Lines       41768    44524    +2756     
==========================================
+ Hits        33846    34357     +511     
- Misses       7922    10114    +2192     
- Partials        0       53      +53     

Flag	Coverage Δ
unittest	80.97% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
nexus/internal/debounce/debounce.go	100.00% <100.00%> (ø)
nexus/pkg/server/sender.go	17.06% <0.00%> (ø)

... and 44 files with indirect coverage changes