fix: wrap the Parse error in UUID.Scan with %w

[januththedev]

Summary

UUID.Scan flattened the Parse error with %v, discarding its identity so errors.Is/errors.As could not classify it.

u, err := Parse(src)
if err != nil {
    return fmt.Errorf("Scan: %v", err)   // loses the wrapped error
}

PR #166 (shipped in v1.6.0) added ErrInvalidLength, ErrInvalidUUIDFormat, ErrInvalidURNPrefix, ErrInvalidBracketedFormat and IsInvalidLengthError() specifically so callers could use errors.Is, and it wrapped Parse, ParseBytes and Validate with %w. UUID.Scan — the primary entry point for reading UUIDs out of a SQL driver — was missed, and sql.go:29 is the only fmt.Errorf in the package that flattens a real error. The []byte branch recurses into the string branch, so it is affected too, as is NullUUID.Scan which delegates.

Note the error text was already correct. Only the error type was lost, which is why this has gone unnoticed.

Reproduction

_, err := uuid.Parse("12345")
_, scanErr := (&uuid.UUID{}).Scan("12345")
fmt.Println(errors.Is(scanErr, uuid.ErrInvalidLength))   // false, should be true

Parse err=invalid UUID length: 5
Scan  err=Scan: invalid UUID length: 5
IsInvalidLength(ScanErr)=false

The change

return fmt.Errorf("Scan: %w", err)

Tests

TestScanErrorWrapping is table-driven with six subtests covering string and []byte inputs for all three error types. Each asserts both that errors.Is now matches the sentinel and that the message text is byte-identical to before, which is what demonstrates the change is non-breaking.

• Before: 6/6 subtests fail. After: 6/6 pass.
• go test -count=1 ./... → 0 failures, 173 subtests passing.
• go vet ./... clean.

The only formatting note is that gofmt -l flags most files in this checkout, which is pre-existing CRLF line endings from Windows; a pristine clone produces the identical list.

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