[Subscription] Disable subscription by default

[Caideyipi]

Description

Disable subscription by default and expose subscription_enabled=false in the system configuration template.

When subscription is disabled, its query-memory proportion is forced to zero, so the configured subscription share remains available to the other query-engine components. Subscription integration tests now enable the feature explicitly.

Tests

• mvn spotless:check -pl iotdb-core/node-commons,iotdb-core/datanode -Dcheckstyle.skip=true
• mvn spotless:check -pl integration-test -Pwith-integration-tests -Dcheckstyle.skip=true
• mvn -pl iotdb-core/node-commons -Dtest=CommonConfigTest -DfailIfNoTests=false -Drat.skip=true -Dcheckstyle.skip=true -DforkCount=0 test
• DataNodeMemoryConfigTest via isolated JUnit execution: 12 tests passed

The regular DataNode Maven test command is currently blocked by unrelated generated-source and local snapshot mismatches, including missing Accumulator, IFill, and a stale ConfigNode Thrift interface.

---

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.

---

Key changed/added classes (or packages if there are too many classes) in this PR

• CommonConfig
• DataNodeMemoryConfig
• Subscription integration-test configuration