fix a possible use after free in StorageKafka2::activate()

[bharatnc]

When the ZooKeeper session expires, partialShutdown was not resetting replica_is_active_node, so the old EphemeralNodeHolder would get destroyed after setZooKeeper had already freed the old session, causing a segfault.

Closes: #97495

Changelog category (leave one):

• Critical Bug Fix (crash, data loss, RBAC) or LOGICAL_ERROR

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Fix a possible use after free in StorageKafka2.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [a2b94f6]

Summary: ❌

job_name	test_name	status	info	comment
BuzzHouse (amd_ubsan)		failure
	Logical error: Digest does not match (STID: 5029-4dec)	FAIL	cidb, issue
Finish Workflow		failure
	python3 ./ci/jobs/scripts/workflow_hooks/new_tests_check.py	failure

[bharatnc]

Failure Finish Workflow

 Run command [gh pr view 97520 --json title,body,labels --repo ClickHouse/ClickHouse]
No new tests have been added

But I don't think it's easy to add a test for this.

[alexey-milovidov]

I've relaxed this rule: #97565
Only need to provide a link to CI report.