fix(logging): preserve Unicode prefix spacing

[Ricardo-M-L]

Problem

LogToFileWith promises to append an ASCII space only when the supplied prefix does not already end in whitespace. It currently inspects the final byte and converts that byte to a rune:

finalChar := prefix[len(prefix)-1]
unicode.IsSpace(rune(finalChar))

That works for ASCII but misreads every multibyte trailing rune. Prefixes ending in valid Unicode whitespace such as U+00A0 NO-BREAK SPACE or U+3000 IDEOGRAPHIC SPACE therefore receive an unwanted extra ASCII space.

Fix

Decode the final UTF-8 rune before applying unicode.IsSpace. ASCII behavior stays unchanged, while multibyte whitespace now follows the documented rule.

The table-driven regression test covers empty prefixes, missing and existing ASCII spaces, U+00A0, and U+3000.

Validation

• go test -run TestLogToFile -count=100 ./...

• go test -race -run TestLogToFile -count=20 ./...

• go test ./...

• go test -race -count=1 ./...

• go vet ./...

• go test ./... in examples/

• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run

• git diff --check

• I have read CONTRIBUTING.md.

[Ricardo-M-L]

Friendly ping — this is a minimal cosmetic fix preserving Unicode prefix spacing in log output. No functional changes. Any feedback welcome!