Add checksum JSON validation tests

[modernzju]

Summary

This PR adds focused tests for the types.Checksum JSON behavior and constructor validation.

The new coverage verifies that:

• valid checksums round-trip through JSON as hex strings
• invalid checksum JSON is rejected for malformed hex, wrong length, and non-string input
• ForceNewChecksum panics for invalid input as documented

Motivation

Checksum is used as the Go representation of Wasm code hashes and has custom JSON marshal/unmarshal logic with a strict 32-byte length requirement. These tests make that public behavior explicit and help prevent regressions around checksum serialization at the Go/Rust boundary.

Testing

go test ./types -count=1
CGO_ENABLED=0 go test ./types -count=1
go test ./... -count=1
git diff --check