Fix currentSize calculation in BulkIngester

[gaobinlong]

Description

currentSize in BulkIngester tracks the total byte size of buffered operations, used by canAddOperation() to enforce the maxSize flush threshold. After each flush, line 381 reset it incorrectly:

// Before — wrong: operations.size() returns a count (e.g. 3), not bytes (e.g. 30,000)
currentSize = operations.size();

This caused canAddOperation() to compare bytes against a count, making the maxSize-based flush trigger effectively broken — the buffer would almost never flush based on byte size alone.

The fix has two parts:

1. Make IngesterOperation generic with Context

IngesterOperation previously used a raw RetryableBulkOperation field, causing unchecked cast warnings and requiring callers to go through .repeatableOperation(). It is now IngesterOperation with typed fields and delegation methods (operation(), context(), isSendable(),canRetry(), retries()).

2. Store IngesterOperation in the operations list

Previously the list stored RetryableBulkOperation (without size). By storing IngesterOperation instead, each entry carries its precomputed byte size. currentSize is now decremented inline as operations are removed during flush:

// After — correct: subtract the actual byte size of each sent operation
currentSize -= op.size();

Issues Resolved

#2112

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[reta]

Thanks @gaobinlong , this is cherrypick of elastic/elasticsearch-java#1167, right?