test: probe the Codecov setup with uncovered source

[Jozsef Damokos (jozsefdamokos)]

Probe for #68 — do not merge. Based on that branch, not on trunk.

Adds src/CoverageProbe.php: ~20 uncovered lines with no test, so the upload carries a patch at 0% coverage.

What it can prove now:

• the Upload coverage to Codecov step runs on exactly one leg (mysql:8.0 / PHP 8.3) and the upload is accepted — which is also the first real test of whether the organisation CODECOV_TOKEN is a global upload token rather than the platform repo's own;
• codecov/project and codecov/patch appear as commit statuses;
• the reported paths are src/CoverageProbe.php and not custom/plugins/SwagMigrationMagento/src/CoverageProbe.php — if they are nested, .github/codecov.yml needs a fixes: entry.

What it cannot prove yet: that a coverage drop turns the status red. Codecov has never received a report for this repository, so there is no trunk baseline to compare against and both statuses may come back neutral on a first upload. That part is only demonstrable once #68 has merged and one push: trunk run has established the baseline — at which point this branch can be re-run.

Part of shopware/shopware#18667.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.75%. Comparing base (a5d8e1d) to head (367c321).

Files with missing lines	Patch %	Lines
src/CoverageProbe.php	0.00%	20 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##             fix/phpunit-bootstrap-exit-code      #69      +/-   ##
=====================================================================
- Coverage                              34.83%   34.75%   -0.09%     
- Complexity                              2337     2346       +9     
=====================================================================
  Files                                    437      438       +1     
  Lines                                   8561     8581      +20     
=====================================================================
  Hits                                    2982     2982              
- Misses                                  5579     5599      +20     

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