refactor: restore DepartmentName enum and add FixtureMonkey-based member registration tests

[hyunbin1]

Motivation

• Previous change added a College field to DepartmentName which would require updates across domain logic, so the enum was restored to its original single-argument form to avoid wide ripples.
• Test data generation and combination logic were previously scattered or hardcoded, making maintenance difficult when adding/removing departments.
• Use FixtureMonkey to centralize and simplify creation of valid and invalid request DTOs so tests focus on verification rather than data construction.

Description

• Reverted DepartmentName to the original signature with a single label field to keep domain compatibility (src/main/java/.../DepartmentName.java).
• Added CollegeDepartmentTestData to hold test-only mappings of College -> valid DepartmentName sets and to provide a stream of valid pairs (src/test/java/.../CollegeDepartmentTestData.java).
• Added MemberRegistrationRequestFixture using FixtureMonkey to produce valid and invalid MemberRegistrationRequestDTO instances for tests (src/test/java/.../MemberRegistrationRequestFixture.java).
• Added MemberControllerRegistrationIntegrationTest as a parameterized WebMvc test that iterates valid college/department pairs to assert 201 Created and a negative case asserting 400 Bad Request for invalid student number format (src/test/java/.../MemberControllerRegistrationIntegrationTest.java).
• Kept FixtureMonkey as a test dependency in build.gradle so tests can use FixtureMonkey builders.

Testing

• Ran ./gradlew --version (with JAVA_HOME set to a Java 17 installation) to ensure Gradle and JVM compatibility, which completed successfully.
• Attempted ./gradlew test --tests "*MemberControllerRegistrationIntegrationTest" (with JAVA_HOME set to Java 17) and the test run failed because Gradle could not resolve the org.springframework.boot plugin due to network restrictions (HTTP 403) when accessing plugin/maven repositories, so the integration tests could not be executed in this environment.

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