[fix][ml] Fix backlog estimation during ledger rollover with pending writes

[Denovo1998]

Follow-up to #26184.

Motivation

After ledger rollover, a caught-up cursor can move to currentLedger:-1 while the first write to the new ledger is still pending. Since currentLedgerEntries includes pending writes, the check introduced in #26184 can reject this valid state and cause getEstimatedSizeSinceMarkDeletePosition() to throw, potentially skipping a compaction check.

The same check also incorrectly accepts nonnegative mark-delete entry IDs on an empty current ledger.

Modifications

• Recognize the exact currentLedger:-1 sentinel without relying on currentLedgerEntries.
• Snapshot the current ledger handle before checking its ID.
• Add two regression tests to ManagedLedgerBkTest, using real BookKeeper operations without mocks or direct internal-state mutation. Cover a pending first write during rollover and an invalid nonnegative mark-delete entry ID on an empty ledger.
• Verify that backlog estimation and reading work after the pending write completes.

Verifying this change

• Make sure that the change passes the CI checks.

Both new regression tests fail on the original implementation for the expected reasons. After the fix, all 9 selected tests and quickCheck pass locally:

./gradlew :managed-ledger:test \
  --tests 'ManagedLedgerBkTest.testEstimatedUnackedSize*' \
  --tests 'ManagedCursorTest.testEstimated*' \
  -PtestRetryCount=0 -PtestFailFast=false

./gradlew quickCheck

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

[lhotari]

LGTM. Thanks for fixing the rollover case and adding regression coverage for both pending writes and invalid positions.