TUL/Authorization redirect fix

[milanmajchrak]

Phases	MP	MM	MB	MR	JM	Total
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Developing	0	0	0	0	0	0
Review	0	0	0	0	0	0
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ETA cust.	-	-	-	-	-	0

Problem description

[Copilot]

Pull Request Overview

This PR adds a null check for the result of converter.toRest() in the identifier lookup endpoint to handle cases where users lack permissions to access the resolved DSpace object. When the converter returns null due to authorization failures, the endpoint now returns a 401 Unauthorized error instead of attempting to use the null object.

Key Changes

• Added null check after calling converter.toRest() to prevent NullPointerException when authorization fails
• Returns HTTP 401 Unauthorized with "Authentication is required" message when conversion returns null

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[Copilot]

The error response incorrectly assumes the user is unauthenticated when toRest() returns null. According to ConverterService.toRest() (lines 115-126), null is returned when authorization fails, which can occur for both authenticated and unauthenticated users. The standard DSpace pattern (see DSpaceApiExceptionControllerAdvice lines 75-84) is to check context.getCurrentUser() and return SC_FORBIDDEN (403) with 'Access is denied' if the user is authenticated, or SC_UNAUTHORIZED (401) with 'Authentication is required' if the user is not authenticated. This should be: if (context.getCurrentUser() != null) { response.sendError(HttpServletResponse.SC_FORBIDDEN, \"Access is denied\"); } else { response.sendError(HttpServletResponse.SC_UNAUTHORIZED, \"Authentication is required\"); }

Suggested change

	response.sendError(HttpServletResponse.SC_UNAUTHORIZED, "Authentication is required");
	if (context.getCurrentUser() != null) {
	response.sendError(HttpServletResponse.SC_FORBIDDEN, "Access is denied");
	} else {
	response.sendError(HttpServletResponse.SC_UNAUTHORIZED, "Authentication is required");
	}