fix: logic for appending api returned validators

[drohit-cb]

What changed? Why?

This PR helps fix the logic for handling validators returned from the backend.

Testing

1. Tested with this example script

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/coinbase/coinbase-sdk-go/pkg/coinbase"
)

func main() {
	ctx := context.Background()
	client, err := coinbase.NewClient(
		coinbase.WithAPIKeyFromJSON("api-key.json"),
	)
	if err != nil {
		log.Fatalf("error creating coinbase client: %v", err)
	}

	validators, err := client.ListValidators(ctx, "ethereum-holesky", coinbase.Eth)
	if err != nil {
		log.Fatalf("error listing rewards: %v", err)
	}

	for _, validator := range validators {
		fmt.Println(validator.ToString())
	}

	validator, err := client.GetValidator(ctx, "ethereum-holesky", coinbase.Eth, "my-validator")
	if err != nil {
		log.Fatalf("error getting validator: %v", err)
	}

	fmt.Printf("\nValidator id [%s] has state: %s", validator.ID(), validator.Status())
}

2. Added unit tests to cover 100% of validator.go

Qualified Impact

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 2/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[deangalvin-cb]

@drohit-cb you can update the changelog with the version 0.0.2, and cut a tag for this! :D