Dispose image URL validation requests and responses

[Weegley]

Summary

Dispose HttpRequestMessage and HttpResponseMessage instances created while validating remote image URLs.

This prevents connections from remaining in CLOSE_WAIT during metadata and image refreshes.

Changes

• Dispose each image validation request.
• Dispose each image validation response.
• Add a regression test covering request and response disposal.

Testing

• All 13 automated tests pass.
• Tested with Jellyfin 10.11.11.
• Performed a full metadata and image refresh for approximately 167 movies.
• Tested newly added movies and repeated metadata replacement.
• ESTABLISHED connections closed normally.
• CLOSE_WAIT remained at or near zero.
• No RemoteImageUrlSanitizer HTTP timeouts occurred.