[fix][common] Handle synchronous failures from future suppliers

[Radiancebobo]

Motivation

Some asynchronous helper methods accept a Supplier<CompletableFuture<T>> and assume the supplier always returns a future. However, a supplier can also fail synchronously before creating the future.

Before this change, those synchronous failures could bypass the expected asynchronous error handling path:

• FutureUtil.Sequencer.sequential could throw directly instead of returning a failed future.
• FutureUtil.composeAsync could throw inside the executor task and leave the returned future incomplete.
• Futures.executeWithRetry in managed-ledger could throw before entering the retry state machine, so a retryable transient failure from the operation setup path would not be retried.

These helpers should preserve the CompletableFuture contract: callers should receive a completed or exceptionally completed future, rather than having synchronous exceptions escape or leave the returned future hanging.

Modifications

This change updates the future supplier handling paths to convert synchronous supplier failures into failed futures:

• Added a private helper in FutureUtil to safely invoke Supplier<CompletableFuture<T>>.
• Updated FutureUtil.Sequencer.sequential to use the safe supplier invocation path.
• Updated FutureUtil.composeAsync to complete the returned future exceptionally when the supplier fails synchronously.
• Updated managed-ledger Futures.executeWithRetry to route synchronous supplier failures through the existing retry logic.
• Added null-future handling so suppliers that return null complete exceptionally with NullPointerException instead of causing an unchecked synchronous failure.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Added FutureUtilTest.testSequencerReturnsFailedFutureWhenTaskThrowsSynchronously
• Added FutureUtilTest.testComposeAsyncReturnsFailedFutureWhenSupplierThrowsSynchronously
• Added FuturesTest.testExecuteWithRetryHandlesSynchronousFailure

Local verification:

• ./gradlew :pulsar-common:test --tests org.apache.pulsar.common.util.FutureUtilTest -PtestRetryCount=0 --rerun-tasks
• ./gradlew :managed-ledger:test --tests org.apache.bookkeeper.mledger.util.FuturesTest -PtestRetryCount=0 --rerun-tasks

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

If the box was checked, please highlight the changes

• 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

[Radiancebobo]

There's a similar PR #25931

[Radiancebobo]

/pulsarbot rerun

[Radiancebobo]

I have executed these and made sure there are no issues.

./gradlew :pulsar-common:test :managed-ledger:test --rerun-tasks -PtestRetryCount=0 \
  --tests org.apache.pulsar.common.util.FutureUtilTest \
  --tests org.apache.bookkeeper.mledger.util.FuturesTest

./gradlew :pulsar-common:spotlessCheck :pulsar-common:checkstyleMain :pulsar-common:checkstyleTest