Add check to verify existence of image at image_uri in Satellite Model registration

[NiveditJain]

Currently, the Satellite Model includes the following field:

image_uri: str = Field(..., description="OCI/Docker image URI for the satellite")

When a satellite is registered via the new process (see PR #47), there is no verification that the image referenced by image_uri actually exists.

Feature Request:

• Add logic to validate that the provided image_uri points to an existing image (OCI/Docker).
• If the image does not exist, registration should fail with a clear error message.

This will help avoid invalid or mistyped image URIs and improve reliability of the registration workflow.

[bhaveshAswani112]

Image_uri field is optional in the Satellite model, so I think it shouldn't cause any issues if the image is not provided. If I am wrong, can you please elaborate the issue?

[NiveditJain]

@bhaveshAswani112 its not about the optional field, its about when the value of field is provided we need to quickly check with a low resource call that image is actually accessible.