PYTHON-6052 - Fix TEST_ARGS splitting in coverage runs

[NoahStapp]

PYTHON-6052

Changes in this PR

See ticket.

Test Plan

N/A

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

Pull request overview

Fixes how TEST_ARGS are passed when running tests under coverage in Evergreen so that arguments containing spaces (e.g., marker expressions constructed by handle_green_framework()) are not broken by string/shell-style splitting.

Changes:

• Removes shlex-based command construction for coverage runs and passes argv as a list to subprocess.run.
• Updates coverage execution to run coverage run -m pytest with TEST_ARGS and sys.argv[1:] preserved as distinct argv items.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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