Add test coverage for deleteAssignment methods

[Copilot]

The deleteAssignment methods (both Integer and String overloads) lacked explicit test coverage, inconsistent with other writer operations like createAssignment which have comprehensive tests.

Changes

• AssignmentWriterUTest.java: New test class covering both deleteAssignment(String, Integer) and deleteAssignment(String, String) methods
• DeletedAssignment.json: Sample response data for delete operations

Tests

// Tests deletion with numeric ID
assignmentWriter.deleteAssignment("1234", 123);

// Tests deletion with LTI context ID
assignmentWriter.deleteAssignment("1234", "lti_context_id:ab84f579-4442-4d4a-acd8-85c5ec6fd2b6");

// Tests deletion with numeric string ID
assignmentWriter.deleteAssignment("1234", "456");

Pattern follows existing writer tests (CalendarWriterUTest, CourseManagerUTest) using FakeRestClient for API mocking.

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