ci: specify junit.xml path for Codecov test results upload

[bearomorphism]

Description

The "Upload test results to Codecov" step in CI intermittently fails with:

info - Found 0 test_results files to report
error - No JUnit XML reports found. Please review our documentation
(https://docs.codecov.com/docs/test-result-ingestion-beta) to generate and upload the file.

The poe cover task (run via poe ci) generates junit.xml in the project root (--junitxml=junit.xml in pyproject.toml line 299), but the codecov/codecov-action@v6 with report_type: test_results relies on auto-discovery which is unreliable.

Fix

Add file: junit.xml to explicitly point the Codecov action at the generated JUnit XML report.

Evidence of flaky behavior

Same branch, same workflow, same step — different outcomes:

Run	Job	"Upload test results to Codecov"	Link
25276231285	python-check (3.10, ubuntu-22.04)	success	link
25276415667	python-check (3.10, ubuntu-22.04)	failure	link

Both runs executed the same workflow without file: specified. The Codecov CLI auto-discovery found junit.xml in one run but not the other. Adding an explicit path eliminates this non-determinism.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes

Generated-by: GitHub Copilot CLI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.10%. Comparing base (b4f4209) to head (78d3486).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## master #1943 +/- ##
=======================================
Coverage 98.10% 98.10% =======================================
Files 61 61 Lines 2748 2748 =======================================
Hits 2696 2696 Misses 52 52 

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[bearomorphism]

Closing - the CI failure was caused by a mypy overload error (now fixed in PR #1941), not a Codecov auto-discovery issue. The JUnit XML was missing because tests never ran due to the lint failure.