fix: stringify *string values in toString

[sb123sb123]

Summary

toString currently formats *string values with fmt.Sprintf("%v", v), exposing the pointer address; a nil *string is rendered as <nil>. Handle *string explicitly so the referenced value is returned, with an empty string for nil pointers.

Fixes #439

Changes

• Add *string handling to strval.
• Add regression coverage for non-nil and nil *string values.

Testing

• go test ./... -run "TestToStringPointers|TestToString" -count=1 � pass

• go vet ./... � pass

• gofmt -w strings.go strings_test.go and git diff --check � pass

• go test ./... -count=1 � the existing TestHtmlDate fails on this Windows runner (2019-06-14 received vs 2019-06-13 expected); the same failure was reproduced with the implementation reverted, so it is unrelated.

• go test -cover . -count=1 � same pre-existing TestHtmlDate failure; coverage reached 84.4%.

• proxy.golang.org was unreachable from the runner, so dependencies were fetched through https://goproxy.cn,direct; no dependency files changed.

• Local validation used the available G-drive Go 1.26.7 toolchain; repository CI remains the coverage for its Go 1.22-1.24 matrix.

AI assistance

This change was prepared with AI assistance from Codex. The implementation, regression test, complete diff, signature, and test results were reviewed before submission.