Fix ImageConverter to properly prepend banners path for all non-HTTP URLs

[Copilot]

The ImageConverter.CheckAndConvertToCompleteUrl method was not correctly handling image paths that contain banners/ but don't start with it. This caused incorrect URL construction for certain TheTVDB image paths.

Problem

When TheTVDB returns paths like v4/series/462199/banners/67eef3239c55e.jpg, the method would incorrectly generate:

https://artworks.thetvdb.com/v4/series/462199/banners/67eef3239c55e.jpg

Instead of the correct:

https://artworks.thetvdb.com/banners/v4/series/462199/banners/67eef3239c55e.jpg

Root Cause

The logic used !input.Contains("banners/") to determine whether to prepend the banners path. This meant that any input already containing banners/ anywhere in the string would skip the prepending, even if it didn't start with banners/.

Solution

Changed the condition to !input.StartsWith("banners/") to ensure the banners/ prefix is always added unless the input already starts with banners/ or is a complete HTTP(S) URL.

Impact

• ✅ Fixes broken image URLs for paths with nested banners directories
• ✅ Maintains all existing behavior for other input types
• ✅ Minimal change with no risk of regression
• ✅ Added verification test to prevent future issues

Testing

All scenarios verified:

• v4/series/462199/banners/67eef3239c55e.jpg → https://artworks.thetvdb.com/banners/v4/series/462199/banners/67eef3239c55e.jpg ✅
• some-image.jpg → https://artworks.thetvdb.com/banners/some-image.jpg ✅
• banners/existing.jpg → https://artworks.thetvdb.com/banners/existing.jpg ✅
• http://external.com/img.jpg → http://external.com/img.jpg ✅

Fixes #78.

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