[fix][test] Avoid blocking common-pool workers in ledger shutdown test

[lhotari]

Motivation

ManagedLedgerFactoryShutdownTest.openEncounteredShutdown delays metadata reads by blocking common-pool workers on a latch. Shutdown also schedules completion on that pool, so a small common pool can stall before the test releases the latch.

Modifications

Gate the mocked metadata reads with a CompletableFuture created for each test invocation. Complete the gate after shutdown, preserving the late-read scenario and callback assertions without occupying workers while waiting.

Verifying this change

• Make sure that the change passes the CI checks.

Verified ManagedLedgerFactoryShutdownTest.openEncounteredShutdown locally with temporary invocationCount = 10: 10/10 passed with retries disabled. With common-pool parallelism set to 2, the original test times out after 5 seconds and the fixed test passes 10/10 invocations. 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