fix: clear inactive result payload when decoding JSON

[jakezwang]

Reusing a Result when decoding JSON leaves the previous inactive payload behind. After an error becomes a success, Error() still returns the old error; after a success becomes an error, OrEmpty() still returns the old value.

Replace the receiver with Ok or Err after successful decoding so both transitions have the same state as a newly constructed result. Regression tests cover both transitions, an empty object, error precedence, same-state updates, and preserving the receiver when decoding fails.

Validation: make build test passed with Go 1.18.10, 1.26.6, and 1.27.1, including the full race suite. make coverage passed with Go 1.27.1; go vet ./... and golangci-lint 2.13.2 also passed on macOS/arm64.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.26%. Comparing base (2cbaa86) to head (005f8fb).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   87.42%   88.26%   +0.83%     
==========================================
  Files          28       28              
  Lines        1774     1772       -2     
==========================================
+ Hits         1551     1564      +13     
+ Misses        217      202      -15     
  Partials        6        6              

Flag	Coverage Δ
unittests	88.26% <100.00%> (+0.83%)	⬆️

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[samber]

Thanks for the fix.