ci: Fix failures in test coverage workflow

[stinodego]

Changes

• Refactor the coverage workflow to upload all reports in a single step. This should be more resilient to spurious errors in the upload step.
• No longer fail the CI if the upload step fails.
• DO fail the CI if running the tests fails
• No longer compute coverage in the regular Python test workflow
• Move codecov.yml to the .github directory

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.98%. Comparing base (575e917) to head (74ee7f9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16083    +/-   ##
========================================
  Coverage   80.97%   80.98%            
========================================
  Files        1386     1386            
  Lines      178439   178465    +26     
  Branches     3075     2877   -198     
========================================
+ Hits       144497   144521    +24     
- Misses      33451    33452     +1     
- Partials      491      492     +1     

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