[Fixes #14095] Upload thumbnails through the UI for the remote spatial datasets

[sijandh35]

Fixes #14095

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[gemini-code-assist]

Code Review

This pull request updates the set_thumbnail method in geonode/resource/manager.py to allow thumbnail assignment when a thumbnail is explicitly provided, regardless of the resource's default capabilities. The review feedback suggests refining the conditional logic to better handle falsy thumbnail values and avoid unintended automatic generation for resources that do not support it.

[Gpetrak]

@sijandh35 although the fix works fine, we could further improve the code in terms of sustainability and readability.
At first I would rename the property from can_have_thumbnail to the can_set_thumbnail and I would move the logic there as below:

def can_set_thumbnail(self, thumbnail_data=None):
    """
   a simple description
    """
    if thumbnail_data:
        return True
    
    # a comment for clarification
    return self.subtype not in {"3dtiles", "cog", "flatgeobuf"}

and then in the manager we could simply have:

if _resource and _resource.can_set_thumbnail(thumbnail):

As I saw the property can_have_thumbnail is used only in this part.