feat: add optional ?server=local/pre_ckan to DELETE /organization/{organization_name}

[rbardaji]

This pull request adds an optional server query parameter to the
DELETE /organization/{organization_name} endpoint, allowing users
to delete organizations from either the local CKAN or a pre-CKAN instance.
It also aligns the test suite with the new behavior, including the
404 status code for non-existent organizations.

Changes

1. Added a ckan_instance parameter in delete_organization to avoid relying
   solely on ckan_settings.ckan.
2. Implemented ?server=local or ?server=pre_ckan in the DELETE /organization
   route.
3. Returns a 400 error if pre_ckan is disabled or missing a valid URL scheme.
4. Returns 404 if the organization does not exist.
5. Updated tests/test_delete_organization.py to:
   • Use ANY to ignore the ckan_instance parameter.
   • Expect a 404 status code for "Organization not found."

Impact

• Users can explicitly select which CKAN instance to delete an organization
  from.
• Maintains backward compatibility by defaulting to local CKAN if the server
  parameter is omitted.
• Provides user-friendly error messages for disabled or misconfigured
  pre_ckan.
• The tests now correctly handle the new 404 vs. 400 behavior.

Testing

• Verified locally that ?server=pre_ckan calls delete_organization with
  pre_ckan, returning 400 if disabled or "No scheme supplied."
• Confirmed that calls without ?server remain on local CKAN.
• The test suite now passes with the updated expectations.