Look up Stringer context values using the original key

[vitalivo]

WithContext formats a fmt.Stringer key before passing it to context.Value, changing its identity. A typed key then returns nil, or a different value stored under its string representation. Use the original key for lookup and its string form only as the output field name.

The regression deliberately stores both keys with different values and fails before the fix. Full Go race tests (95.0% coverage), vet, and golangci-lint pass. Tests run from a path containing github.com/samber/oops, as required by the stacktrace tests and CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.52%. Comparing base (525f106) to head (76fb365).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
+ Coverage   91.20%   91.52%   +0.31%     
==========================================
  Files          15       15              
  Lines        1251     1251              
==========================================
+ Hits         1141     1145       +4     
+ Misses         88       84       -4     
  Partials       22       22              

Flag	Coverage Δ
unittests	91.52% <100.00%> (+0.31%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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

[samber]

Thanks for the fix!