Wrap EOF with helpful message for empty JSON request body

[sh9336]

This PR improves the error returned by ShouldBindJSON when the request
body is empty.

Previously, decoding an empty JSON body returned a plain io.EOF error.
This change wraps io.EOF with a clearer message ("empty request body")
while preserving the original error, so errors.Is(err, io.EOF) continues
to work.

An external test has been added to document and verify this behavior.
Happy to adjust the wording or approach if maintainers prefer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (3dc1cd6) to head (cd9b91b).
⚠️ Report is 230 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4496      +/-   ##
==========================================
- Coverage   99.21%   99.06%   -0.16%     
==========================================
  Files          42       44       +2     
  Lines        3182     2990     -192     
==========================================
- Hits         3157     2962     -195     
- Misses         17       19       +2     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	99.05% <100.00%> (?)
-tags go_json	98.92% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <100.00%> (?)
go-1.25	99.06% <100.00%> (?)
macos-latest	99.06% <100.00%> (-0.16%)	⬇️
ubuntu-latest	98.99% <100.00%> (-0.22%)	⬇️

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