Add reproducer for BufferedChannel partial flush position drift

[yangxianjungree]

Related issue

• Reproducer for DefaultEntryLogger can publish stale entry locations after BufferedChannel partial flush failure #4855.
• This PR intentionally does not fix or close DefaultEntryLogger can publish stale entry locations after BufferedChannel partial flush failure #4855.

Motivation

This draft PR adds a focused reproducer for a BufferedChannel failure mode that can leave the logical BufferedChannel.position() behind the underlying FileChannel.position() after a partial flush failure.

The suspected production impact is stale entry locations and stale ledgersMapOffset being published by DefaultEntryLogger after the same channel is reused.

This is intentionally a reproducer-only draft, not the final fix.

Changes

• Add a fault-injected PartialFailingFileChannel for tests.
• Add a test showing BufferedChannel.position() can lag behind FileChannel.position() after a partial flush IOException.
• Add a test showing BufferedLogChannel.appendLedgersMap() can write a stale ledgersMapOffset after that position drift.

Verification

Passed on release-4.16.7 baseline:

/home/stephen/github/java/pulsar/mvnw -pl bookkeeper-server -Dtest=BufferedChannelTest test
Tests run: 8, Failures: 0, Errors: 0, Skipped: 0

After rebasing onto current branch-4.16, running with -am did not reach tests in this local environment due to a Maven resource copy error in buildtools:

Failed to copy buildtools/src/main/resources/bookkeeper/checkstyle.xml ... Cannot allocate memory

Notes

This PR is intentionally scoped to the minimal unit-level reproducer. Higher-level cluster/E2E reproductions are kept separate from this PR to avoid adding environment-specific setup to the code review.

If maintainers agree with the failure model, the follow-up fix should likely make the main entrylog writer fail closed after non-recoverable write/flush/force failures, and prevent appendLedgersMap() from publishing a stale header offset.