Remove phpdbg from coverage target

[ohmyfelix]

Summary

Switch the coverage Makefile target from phpdbg to php so this repository follows the org-wide coverage runner migration.

Motivation

This repository is part of contributte/contributte#73, which removes phpdbg from coverage targets across Contributte packages.

Changes

• replace -p phpdbg with -p php in both coverage target variants in Makefile
• keep the existing coverage output targets for CI (coverage.xml) and local runs (coverage.html)

Testing

• Tests pass locally (make tests)
• Coverage target passes locally (make coverage)
• CI passes

make coverage currently fails in this environment because PHP CLI has no Xdebug/PCOV extension and coverage is no longer running under PHPDBG.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.83%. Comparing base (09e14dc) to head (ff280bd).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   43.44%   38.83%   -4.62%     
==========================================
  Files          24       24              
  Lines         557      515      -42     
==========================================
- Hits          242      200      -42     
  Misses        315      315              

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