Add Codecov configuration for PR coverage reporting

[astewartau]

Summary

• Adds codecov.yml to configure coverage status checks and PR comments
• Project check: fails if overall coverage is below 80%
• Patch check: requires 80% coverage on new/changed lines
• PR comments show a condensed coverage impact summary, only when coverage changes
• Excludes legacy test_bids_validator.py from coverage reporting (broken imports, never runs as part of test suite)

Test plan

• Verify Codecov PR comment appears on this PR after CI runs
• Verify coverage status checks show on the PR
• Pass 80% coverage gate

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.89%. Comparing base (e2e56d4) to head (804f8c5).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #67       +/-   ##
===========================================
+ Coverage   76.83%   90.89%   +14.06%     
===========================================
  Files          13       13               
  Lines         846      846               
  Branches      124      124               
===========================================
+ Hits          650      769      +119     
+ Misses        169       47      -122     
- Partials       27       30        +3     

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[effigies]

Why omit this?

[bendhouseart]

I think it already is, at least as far as pytest is concerned at line 65:

[tool.pytest.ini_options]
minversion = "6"
testpaths = ["tests"]  # <~~~~~~~~~~~ here
log_level = "INFO"
xfail_strict = true
addopts = ["-ra", "--strict-config", "--strict-markers"]
filterwarnings = ["error"]
norecursedirs = ["data"]
doctest_optionflags = ["NORMALIZE_WHITESPACE", "IGNORE_EXCEPTION_DETAIL"]

So we should either move the test or update toml to point to it no?

[effigies]

Yes, we should do either of those. I don't care which.

This actually demonstrates why I prefer to check coverage of test files, as it showed that it wasn't being run. So let's run it, rather than hide it.

[bendhouseart]

I'm pretty sure the tests are going to fail once these changes are added, but that aside I think these changes would make it good to go.

[effigies]

I can't reproduce these errors locally. Except the one with --pre, since that relaxes the pin and lets chardet get upgraded while requests warns and we error on warnings. What a mess.