Refactor batch retry tests to verify behavior instead of implementation details

[Copilot]

The batch retry tests in PersistentActorUpdateEventProcessorTest were verifying internal method call counts (e.g., insertStatement.bind() called exactly 6 times) rather than actual behavior. This makes tests brittle and prone to breaking when refactoring implementation while preserving functionality.

Changes

• Removed bind() call count assertions from testProcessBatchTooLargeWithTwoEvents, testProcessBatchTooLargeWithThreeEvents, and testProcessBatchTooLargeWithNineEvents
• Tests now verify batch retry mechanism triggers correctly and appropriate execute methods are called with expected statement types
• Created distinct mock instances for each event instead of reusing a single mock, improving test isolation
• Fixed variable shadowing where loop variable event shadowed class field

Before

verify(insertStatement, times(6)).bind("key1", "key2", "actorId", ByteBuffer.wrap(new byte[1024]));

After

verify(cqlSession, times(2)).execute(any(BatchStatement.class));
verify(cqlSession, times(1)).execute(any(BoundStatement.class));

Tests now assert on observable behavior (retry mechanism splits batch and processes events) rather than internal implementation details.

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