fix(math): reset Int state when Unmarshal sees empty input

[sewqasreedas]

• fix the 256-bit Int.Unmarshal implementation so empty byte slices zero the value instead of discarding the receiver pointer
• initialize the internal big.Int when needed, mirroring existing patterns elsewhere in the math package
• surface an explicit error if a nil receiver is passed, avoiding silent corruption

[aljo242]

can you show why this is needed?

[sewqasreedas]

@aljo242 added test TestIntUnmarshalEmpty that demonstrates the bug - when unmarshaling empty bytes, the old code left the value unchanged (12345), while the correct behavior is to reset to zero.

without the fix, the test fails:
--- FAIL: TestIntUnmarshalEmpty (0.00s)
int_test.go:709:
Error: Should be true
Messages: after unmarshaling empty bytes, Int should be zero, but got: 12345

If I had to briefly answer why this is needed: the old code does i = nil which only reassigns a local variable and doesn't actually modify the receiver. This violates the basic Go contract for pointer receiver methods - they must modify the object itself, not reassign the pointer. As a result, calling Unmarshal with empty bytes (which represents field not set in protobuf) leaves the Int with its old value instead of properly resetting to zero.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.50%. Comparing base (bd5f14e) to head (e70c6b1).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
math/int.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25568      +/-   ##
==========================================
+ Coverage   68.39%   70.50%   +2.11%     
==========================================
  Files         856      826      -30     
  Lines       55855    54712    -1143     
==========================================
+ Hits        38201    38576     +375     
+ Misses      17654    16136    -1518     

Files with missing lines	Coverage Δ
math/int.go	88.27% <66.66%> (ø)

... and 44 files with indirect coverage changes

🚀 New features to boost your workflow:

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