feat: Add plan webhook support

[hjk1996]

what

• Added event-aware webhook wiring to support both plan and apply (typed Event + ConfiguredSender, event set on payload, filtered dispatch).
• Updated project runner to send plan/apply webhooks with the correct event and adjusted Slack attachment text to use the event name.
• Extended webhook tests to cover plan configs and event filtering.

why

To deliver plan results through the webhook pipeline, and to make webhook payloads/messages clearly identify which event triggered them.

tests

• Added webhook tests for plan config and event filtering
• Ran locally: go test ./server/events/webhooks (passes)

references

• Add webhook support for atlantis plan results #5707

[abborg]

It might be worth capturing any error that occurs here and emitting an error log.

Suggested change

	webhookSendErr := p.Webhooks.Send(ctx.Log, webhooks.EventResult{ Event: webhooks.ApplyEvent,
	Workspace: ctx.Workspace,
	User: ctx.User,
	Repo: ctx.Pull.BaseRepo,
	Pull: ctx.Pull,
	Success: err == nil,
	Directory: ctx.RepoRelDir,
	ProjectName: ctx.ProjectName,
	})
	if webhookSendErr != nil {
	ctx.Log.Err("error sending apply webhook; %v", webhookSendErr)
	}
	if err != nil {
	return "", "", fmt.Errorf("%s\n%s", err, strings.Join(outputs, "\n"))
	}

[abborg]

Webhook send failures could be emitted as logs, but as it's not currently done for the apply webhook, I don't see it as a blocker for this change.