[fix][test] Await both DLQ producers in multi-consumer test

[lhotari]

Motivation

testDeadLetterTopicWithInitialSubscriptionAndMultiConsumers waits for the dead-letter topic and initial subscription to exist, then immediately asserts that both consumers have a DLQ producer. Each consumer creates its producer asynchronously, so the second producer can still be pending when the topic and subscription are visible.

Modifications

Include the exact two-producer assertion in the existing bounded Awaitility check. The test still verifies that both consumers can create DLQ producers and that all messages reach the dead-letter topic.

Verifying this change

• Make sure that the change passes the CI checks.

Verified DeadLetterTopicTest.testDeadLetterTopicWithInitialSubscriptionAndMultiConsumers locally with temporary invocationCount = 10: 10/10 passed with retries disabled. The temporary invocation count was removed.

./gradlew spotlessCheck checkstyleMain checkstyleTest passed.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[void-ptr974]

LGTM