[DwC export]: Auto-construct Attachment URLs from web asset server URL

[grantfitzsimmons]

Goal

When attachment fields (e.g., aggregated CollectionObjectAttachments) are included in an export mapping, automatically construct valid attachment URLs using the configured web asset server URL and collection.

Background

Attachment URLs are needed for Audiovisual Core and similar extensions but vary by installation. Auto-construction from the configured server URL removes a manual step and avoids errors.

Acceptance Criteria

• If an attachment field is added to a schema mapping, make sure there is an exported value capturing the fully constructed URL (e.g., {webAssetServerURL}/{collection}/{attachmentFilename}).
• The URL is constructed from the web asset server URL configured in the hosted environment.
• No additional user configuration is required beyond including the attachment field.
• If no web asset server URL is configured, a warning is shown and the field exports a blank value.