[fix][broker] Fix BucketDelayedDeliveryTracker recovery after LightProto migration

[nodece]

Motivation

PR #25337 migrated broker delayed-delivery protos from protobuf-java to LightProto. Unlike protobuf-java which eagerly copies bytes fields, LightProto lazily references the parsed ByteBuf. The existing parseSnapshotMetadataEntry released the buffer immediately after parsing, so any later access to delayed_index_bit_map entries (via forEachDelayedIndexBitMap) threw IllegalReferenceCountException (refCnt: 0).

This caused BucketDelayedDeliveryTracker recovery to fail on every broker restart, silently falling back to InMemoryDelayedDeliveryTracker — all bucket snapshots became unreachable, bucket-level metrics disappeared, and new snapshots were no longer created.

Modifications

• Force materialization of delayed_index_bit_map entries into standalone byte[] copies before releasing the entry buffer in parseSnapshotMetadataEntry.
• Add testParseSnapshotMetadataEntryMaterializesLazyBytes that reproduces the bug by mocking LedgerEntry with a refCnt=1 buffer (matching real BK, unlike MockedBookKeeper which masks the bug with an extra reference).

Verifying

The test fails without the fix (IllegalReferenceCountException: refCnt: 0) and passes with it.

[merlimat]

@nodece There's a test failing, related to this change