fix(sdk): fix retry logic in wandb-core and system_tests conftest

[dmitryduev]

Description

• Fixes WB-NNNNN
• Fixes #NNNN

What does the PR do?

• Fixed a gnarly bug in our HTTP retry logic related to time conversions.
• Fixed and cleaned up system_test's conftest, in particular:
  • Moved test server setup to pytest's configure step
  • Fixed broken flag passing introduced in refactor(sdk): Update server testing fixtures and MlFlow tests #5785
  • Removed old fixtures for the (python) service-based internal process

Testing

How was this PR tested?
Manually + go unit tests + system tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5081aed) 79.36% compared to head (baa72d9) 79.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6847      +/-   ##
==========================================
- Coverage   79.36%   79.35%   -0.01%     
==========================================
  Files         452      452              
  Lines       50615    50618       +3     
==========================================
  Hits        40170    40170              
- Misses      10152    10157       +5     
+ Partials      293      291       -2     

Flag	Coverage Δ
func	50.41% <100.00%> (-1.69%)	⬇️
system	64.40% <85.71%> (+0.59%)	⬆️
unit	58.84% <100.00%> (-0.06%)	⬇️

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

Files	Coverage Δ
core/internal/clients/backoff.go	100.00% <100.00%> (ø)
core/internal/clients/clients.go	95.12% <100.00%> (+0.25%)	⬆️
core/pkg/server/sender.go	83.21% <100.00%> (+0.52%)	⬆️
wandb/sdk/wandb_settings.py	93.22% <100.00%> (ø)

... and 33 files with indirect coverage changes