Add tests for carousel template payload validation

[ewilderj]

Description

Add comprehensive tests for carousel template payload validation to ensure invalid payloads are properly rejected with clear error messages.

Work Required

1. Create Test File: Add test/templates.validation.carousel.test.js
2. Test Required Fields: Validate items array (required, must be array, non-empty)
3. Test Bounds: Validate duration bounds (must be positive number within reasonable limits)
4. Test Optional Parameters: Validate zoomScale, zoomMs, zoomOnVideo parameters
5. Follow Existing Pattern: Use same structure as test/templates.validation.api.test.js

Implementation Details

Based on templates/carousel.html, the template expects:

• items: Array of URLs (required, non-empty)
• duration: Number in milliseconds (optional, defaults to 4000)
• zoomScale: Number for zoom effect (optional, defaults to 1.05)
• zoomMs: Duration for zoom animation (optional, defaults to duration)
• zoomOnVideo: Boolean for video zoom (optional)

Test Cases to Cover

• Empty payload {}
• Missing items field
• Empty items array []
• Invalid items type (string, number, etc.)
• Invalid duration values (negative, zero, non-numeric)
• Invalid zoomScale values (negative, extreme values)
• Valid payload with all parameters
• Valid payload with minimal required fields

Files to Create/Modify

• test/templates.validation.carousel.test.js - New test file

Acceptance Criteria

• Invalid payloads are rejected with 400 status and clear error messages
• All validation edge cases are covered
• Tests follow existing patterns in the codebase
• Test coverage includes both positive and negative test cases