Validate IP slice values before appending or replacing

[agammann]

IPSlice rejects invalid addresses through Set, but its SliceValue.Append and SliceValue.Replace methods currently report success and insert nil IPs. A failed replacement also discards the previous valid values.

Make the shared conversion helper return an error for an invalid address. Both operations validate before mutating the destination, matching the other numeric slice implementations. Add tests for invalid append, a partially valid replacement, unchanged state on error, and valid IPv4/IPv6 inputs with surrounding whitespace.

Validation on Windows with Go 1.27.1: both invalid-input regressions fail before the fix; go test ./... and go vet ./... pass after it.

[CLAassistant]

All committers have signed the CLA.

[tomasaschan]

Thanks!