ci: fail-soft on Codecov upload, fix coverage path

[trishorts]

Problem

All three open PRs (#116, #117, #118) fail the single Build and Test
check. The failure is not in any PR's code — restore, build, and tests all
pass (#116 shows 235 passed, 7 skipped, 0 failed across net472 + net6.0).
The only error is the Codecov upload step failing with exit code
4294967295.

Root cause

.github/workflows/dotnet.yml runs codecov/codecov-action@v3 with
fail_ci_if_error: true, so any Codecov upload error fails the entire job
regardless of test results. Two config issues make the upload error likely:

• files: pointed at coverage.net6.0.xml, but the test step writes
  coverage.xml (/p:CoverletOutput='../../coverage.xml'), so the action
  had no file to upload.
• The action ran on the deprecated v3 / Node 20 runner.

Fix

• fail_ci_if_error: true -> false — a Codecov hiccup no longer fails CI.
• Correct the coverage file path to ./coverage.xml.
• Bump checkout, setup-dotnet, and codecov-action to v4 (clears the
  Node 20 deprecation warnings).

Note for maintainers

The underlying Codecov upload error is most likely a missing or expired
CODECOV repo secret (public-repo tokenless uploads get rate-limited).
With fail_ci_if_error: false the build is green either way, but refreshing
that secret would restore actual coverage reporting.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.25%. Comparing base (08dc841) to head (c1935c4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   89.65%   89.25%   -0.41%     
==========================================
  Files         103      103              
  Lines        5702     5721      +19     
  Branches      878      882       +4     
==========================================
- Hits         5112     5106       -6     
- Misses        424      429       +5     
- Partials      166      186      +20     

see 15 files with indirect coverage changes

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