fix(exception-handling): Add ResourceNotFoundException handler for structured error responses [MDD-78]

[devin-ai-integration]

fix(exception-handling): Add ResourceNotFoundException handler [MDD-78]

Summary

Adds explicit @ExceptionHandler for ResourceNotFoundException in both REST and GraphQL exception handlers so that endpoints return structured JSON error responses ({"message": "Resource not found"}) instead of raw/unstructured 404s. This affects all endpoints that throw ResourceNotFoundException (ProfileApi, ArticleApi, CommentsApi, etc.).

Changes:

• CustomizeExceptionHandler: New handler returning 404 with {"message": "Resource not found"}
• GraphQLCustomizeExceptionHandler: New else if branch returning NOT_FOUND typed GraphQL error
• ProfileApiTest: 3 new tests for follow/unfollow/get-profile with nonexistent users, plus CustomizeExceptionHandler added to test @Import

Review & Testing Checklist for Human

• Verify @ResponseStatus vs @ExceptionHandler precedence: ResourceNotFoundException has @ResponseStatus(HttpStatus.NOT_FOUND) on the class. Confirm that the new @ExceptionHandler takes precedence in all cases (it should, but this is the highest-risk interaction). Consider whether the @ResponseStatus annotation should be removed now that there's an explicit handler.
• Confirm no regression on other endpoints: The new handler catches all ResourceNotFoundException usages app-wide (ArticleApi, CommentsApi, ArticleFavoriteApi, GraphQL datafetchers). Verify these endpoints still return 404 as expected with the new structured body.
• GraphQL path is untested: No GraphQL-specific test was added for ResourceNotFoundException. Consider whether a test in the GraphQL layer is needed for confidence.

Suggested manual test plan:

1. Start the app locally
2. POST /profiles/nonexistent-user/follow with auth → expect 404 with {"message": "Resource not found"}
3. DELETE /profiles/nonexistent-user/follow with auth → same
4. GET /profiles/nonexistent-user → same
5. Try a nonexistent article endpoint (e.g. GET /articles/nonexistent-slug) to verify the handler works app-wide

Notes

• Requested by: @milind-cognition
• Link to Devin run

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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