chore: replace go-hex with encoding/hex

[oozan]

What changed

• Replaced the unmaintained github.com/tmthrgd/go-hex dependency with Go's standard encoding/hex package
• Removed go-hex from the root module dependencies
• Added regression coverage for empty, single, and incremental writes

The two packages expose compatible EncodedLen and Encode functions, so SQL byte encoding behavior remains unchanged while removing an abandoned supply-chain dependency.

Testing

• go test ./internal ./schema
• go vet ./internal ./schema
• git diff --check

Fixes #1385

AI assistance was used to prepare this focused change. I reviewed the implementation and test coverage and am prepared to address review feedback.