kafka/tests: fix flaky unlicensed enterprise tests

[nguyen-andrew]

The unlicensed_reject_defaults and unlicensed_rejected tests in create_topics_test.cc are flaky in CI. Two issues cause the tests to intermittently pass when they should enforce enterprise license checks:

1. A race between revoke_license() and the async trial license init metrics_reporter::try_initialize_cluster_info(). If the init completed before revoke, should_sanction() is permissive.
2. lconf().cloud_storage_enabled.set_value(true) only sets the config shard 0. If the Kafka handler runs on another shard, is_restricted() returns false.

This adds a wait_for_license_init() helper that waits for the async trial license to be initialized before revoking, and switches config mutations to update_cluster_config() to propagate to all shards.

Fixes CORE-15319

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

• none

[nguyen-andrew]

/rp-unit-test
debug
arch=arm64
ctest_args=-R create_topics_test_rpunit --timeout 1000 --repeat until-fail:500

[vbotbuildovich]

Invalid arch arm64 . Must be one of amd64,arm64.

Workflow run logs.

[nguyen-andrew]

/rp-unit-test
debug
arch=arm64
ctest_args=-R create_topics_test_rpunit --timeout 1000 --repeat until-fail:500