fix(coverage): collect coverage from multiprocess child processes

[vringar]

Summary

• OpenWPM uses the third-party multiprocess library (a dill-based fork of stdlib multiprocessing), so coverage.py's built-in concurrency and patch settings don't instrument child processes
• Additionally, multiprocess uses os._exit() after Process.run(), which skips atexit handlers and prevents coverage from saving data
• Adds coverage.process_startup() and explicit cov.stop()/cov.save() in the Process wrapper, using a template method pattern (run/run_impl) so the logic lives in one place
• Adds a pytest fixture to set COVERAGE_PROCESS_START and COVERAGE_FILE env vars during coverage runs
• BrowserManager now uses the common Process wrapper and overrides run_impl() instead of run()

Fixes #1121

[Copilot]

Pull request overview

This PR implements coverage collection for child processes spawned using the third-party multiprocess library. Since OpenWPM uses multiprocess (a dill-based fork of stdlib multiprocessing) rather than the standard library, coverage.py's built-in concurrency settings don't automatically instrument child processes. Additionally, multiprocess uses os._exit() which bypasses atexit handlers, preventing coverage data from being saved normally.

Changes:

• Added a pytest fixture to set COVERAGE_PROCESS_START and COVERAGE_FILE environment variables during coverage runs
• Implemented a template method pattern in the Process wrapper, with run() handling coverage startup/shutdown and subclasses overriding run_impl()
• Updated BrowserManager to use the common Process wrapper and override run_impl() instead of run()

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/conftest.py	Added session-scoped autouse fixture to configure coverage environment variables for child processes
openwpm/utilities/multiprocess_utils.py	Implemented template method pattern with coverage.process_startup() at the start and explicit cov.stop()/cov.save() in finally block; added run_impl() method for subclasses
openwpm/browser_manager.py	Changed BrowserManager to import Process from multiprocess_utils and renamed run() to run_impl() to work with the template method pattern

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[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.24%. Comparing base (8be0aeb) to head (792ea97).

Files with missing lines	Patch %	Lines
openwpm/utilities/multiprocess_utils.py	16.66%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1123       +/-   ##
===========================================
+ Coverage   37.32%   56.24%   +18.91%     
===========================================
  Files          35       36        +1     
  Lines        3475     3540       +65     
===========================================
+ Hits         1297     1991      +694     
+ Misses       2178     1549      -629     

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