Fix TestCampaign sending only subscribers, failing listmonk validation

listmonk's /api/campaigns/:id/test handler binds the request into the
same campReq struct create/update use and validates it in full, so a
request with only {"subscribers": [...]} fails with "Invalid length
for name" before subscribers is ever read. TestCampaign now sends the
campaign's full CampaignInput alongside subscribers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/listmonk/listmonk.go

@@ -297,6 +297,23 @@ type Campaign struct {
 	MediaIDs   []int
 }
 
+// AsInput converts a fetched Campaign back into the CampaignInput shape,
+// for callers (e.g. Test) that need to re-send a campaign's current fields
+// against an endpoint that validates the full campaign body.
+func (camp *Campaign) AsInput() CampaignInput {
+	return CampaignInput{
+		Name:       camp.Name,
+		Subject:    camp.Subject,
+		FromEmail:  camp.FromEmail,
+		TemplateID: camp.TemplateID,
+		Type:       camp.Type,
+		Tags:       camp.Tags,
+		ListIDs:    camp.ListIDs,
+		Body:       camp.Body,
+		MediaIDs:   camp.MediaIDs,
+	}
+}
+
 // CampaignInput is what sync.go builds from a campaign's frontmatter+body to
 // create or update a listmonk campaign.
 type CampaignInput struct {
@@ -482,14 +499,18 @@ func (c *Client) SetCampaignStatus(id int, status string) error {
 }
 
 // TestCampaign sends a preview of the campaign's current content to the
-// given addresses without touching its status. Request field confirmed
-// against knadh/listmonk's actual handler source (cmd/campaigns.go's
-// campReq.SubscriberEmails, json tag "subscribers").
-func (c *Client) TestCampaign(id int, emails []string) error {
+// given addresses without touching its status. listmonk's test-send handler
+// binds the request into the same campReq struct create/update use and
+// validates it in full, so the campaign's other fields (in particular a
+// non-empty "name") must be sent alongside "subscribers" — a request with
+// just {"subscribers": [...]} fails listmonk's own validation with "Invalid
+// length for name" before the "subscribers" field is ever looked at.
+func (c *Client) TestCampaign(id int, in CampaignInput, emails []string) error {
 	if len(emails) == 0 {
 		return nil
 	}
-	payload := map[string]any{"subscribers": emails}
+	payload := in.payload()
+	payload["subscribers"] = emails
 	respBody, status, err := c.do(http.MethodPost, fmt.Sprintf("/api/campaigns/%d/test", id), payload)
 	if err != nil {
 		return fmt.Errorf("sending test for campaign %d: %w", id, err)