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 70.47%. Comparing base (4c28078) to head (0906e8f).
⚠️ Report is 1 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   70.29%   70.47%   +0.18%     
==========================================
  Files         821      826       +5     
  Lines       53686    54630     +944     
==========================================
+ Hits        37740    38502     +762     
- Misses      15946    16128     +182     

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

... and 6 files with indirect coverage changes

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