fix(contrib/registry/eureka): improve server picking and retry logic

[1911860538]

This PR fixes potential issues in the Eureka client's server picking and retry logic.
e.maxRetry is not necessarily equal to len(e.urls),
because users can customize e.maxRetry using the WithMaxRetry function.
When users set e.maxRetry=0, the for-loop in the Client.do method will not execute.
When e.maxRetry is greater than len(e.urls), array index out of bounds may occur.
For example, if a user sets e.maxRetry=10 and len(e.urls)=2,
when pickServer is executed with currentTimes=2,
currentTimes%e.maxRetry=2, resulting in an array index out of bounds.

[1911860538]

Reopen #3771
On the main branch, the following code can reproduce the array out-of-bounds issue.

kratos/contrib/registry/eureka/client_test.go

package eureka

import (
	"context"
	"testing"
)

// This test demonstrates an out-of-bounds panic in pickServer
// when maxRetry > len(urls).
func TestPickServer_OutOfBoundsPanic(t *testing.T) {
	// len(urls) = 2
	e := NewClient(
		[]string{
			"http://127.0.0.1:8080",
			"http://127.0.0.1:8081",
		},
		WithMaxRetry(3), // > len(urls)
	)

	// This call will eventually invoke pickServer with currentTimes = 2,
	// which leads to e.urls[2%3] == e.urls[2] and panics:
	// runtime error: index out of range [2] with length 2
	_ = e.Out(context.Background(), "app", "instance")
}