test: install goravel/slack and add Slack notification tests

[goravel-coder]

Summary

• Install and register the github.com/goravel/slack package, exposing a slack notification channel configured via SLACK_BOT_TOKEN/SLACK_CHANNEL, with the target channel read through the app config (slack.channel) so .env values are honored.
• Add a live Slack notification test suite covering channel registration plus text and rich (attachment/field) delivery to a real workspace, skipped locally when no bot token is set; send failures now report the target channel.
• Add package install/uninstall round-trip coverage and wire the Slack bot token and channel into feature CI via secrets.

Why

The example app previously had no Slack notification channel: facades.Notification().Channel("slack") resolved to nil and Slack delivery couldn't be demonstrated. This change installs and registers the github.com/goravel/slack package so notifications can be routed to a real Slack workspace, and adds tests proving the channel is registered, text and rich messages are delivered, and the package can be cleanly installed/uninstalled.

The target channel is read through facades.Config().GetString("slack.channel") (backed by SLACK_CHANNEL) rather than os.Getenv, so a channel set in .env is respected and the same source is used across the suite. The live-send assertions now include the channel in their error message, and feature CI passes SLACK_BOT_TOKEN/SLACK_CHANNEL through as secrets.

import (
	"github.com/goravel/framework/contracts/notification"
	slackcontracts "github.com/goravel/slack/contracts"
)

type InvoicePaid struct {
	Amount string
}

func (r *InvoicePaid) Via(notifiable notification.Notifiable) []string {
	return []string{slackcontracts.ChannelName}
}

func (r *InvoicePaid) ToSlack(notifiable notification.Notifiable) slackcontracts.Message {
	return slackcontracts.Message{
		Text: "Invoice paid: $" + r.Amount,
	}
}

// send it
facades.Notification().
	Route(slackcontracts.ChannelName, "#sales").
	NotifyNow(&notifications.InvoicePaid{Amount: "99.00"})