feat(cassandra): modernize test infrastructure with Spring Boot Testcontainers

[CharlesWong]

Problem

The existing Cassandra test infrastructure used a custom ContextCustomizerFactory pattern (CassandraTestContainersSpringContextCustomizerFactory) that:

• Used deprecated Cassandra driver 3.x API (getCluster().getMetadata().getAllHosts())
• Registered via spring.factories (old approach)
• Was inconsistent with how other databases (MongoDB, Couchbase, Neo4j) are handled in JHipster

This was the original issue behind #21466 (CassandraUnit was unmaintained; Testcontainers was introduced but used the old integration pattern).

Solution

Modernize to use Spring Boot's native Testcontainers support, consistent with MongoDB/Couchbase/Neo4j:

1. CassandraTestContainer is now an interface with @Container + @DynamicPropertySource (same pattern as MongoDbTestContainer)
2. IntegrationTest uses @ImportTestcontainers(CassandraTestContainer.class) instead of @EmbeddedCassandra
3. spring-boot-testcontainers module added for Spring Boot auto-configuration
4. CassandraTestContainersSpringContextCustomizerFactory removed
5. EmbeddedCassandra annotation removed
6. Testcontainers artifact corrected to org.testcontainers:cassandra
7. cleanup.ts updated to remove old files on upgrade

Changes

• generator.ts: Add spring-boot-testcontainers module; fix Testcontainers artifact to cassandra; use @ImportTestcontainers instead of old @EmbeddedCassandra; remove addTestSpringFactory call
• files.ts: Remove CassandraTestContainersSpringContextCustomizerFactory.java and EmbeddedCassandra.java from generated files
• cleanup.ts: Add cleanup for removed files when upgrading from < 9.1.0
• CassandraTestContainer.java.ejs: Rewritten as interface with @Container + @DynamicPropertySource; uses modern driver 4.x API
• CassandraTestContainersSpringContextCustomizerFactory.java.ejs: Deleted
• EmbeddedCassandra.java.ejs: Deleted

Before / After

Before (IntegrationTest.java)

@EmbeddedCassandra
@SpringBootTest
public @interface IntegrationTest { ... }

After (IntegrationTest.java)

@ImportTestcontainers(CassandraTestContainer.class)
@SpringBootTest
public @interface IntegrationTest { ... }

Benefits

• ✅ Consistent with JHipster's existing Testcontainers pattern (MongoDB, Couchbase, Neo4j)
• ✅ Works with reactive Spring Data Cassandra
• ✅ Uses modern DataStax OSS driver 4.x API
• ✅ No more spring.factories registration needed
• ✅ Simpler, less code

Closes #21466