Fix Codecov CLI publishing

[AArnott]

The retired Codecov uploader endpoints now return 404, while the workflow masked upload failures. This migrates coverage publishing to the supported Codecov CLI and makes upload failures fail the build.

• Download the platform-specific CLI from cli.codecov.io into a distinct cache, using the current Codecov signing key and explicit GPG failure checks while retaining signature and SHA verification.
• Invoke upload-process with explicit file discovery disabled, fail-on-error enabled, and optional flag/name arguments only when provided.
• Propagate the uploader's exact nonzero exit code and remove continue-on-error from the workflow step.

Validation included PowerShell parsing, a fresh signed CLI download and SHA verification, required upload-process argument checks, and exact propagation of exit code 37 from a fake uploader.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (dba613e) to head (e302561).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #547   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            2         2           
=========================================
  Hits             2         2           

Flag	Coverage Δ
Linux	50.00% <ø> (ø)
Windows	100.00% <ø> (ø)
macOS	50.00% <ø> (ø)

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