Issue #13513 - Refactor MemoryEndPointPipe to use RetainableByteBuffer.DynamicCapacity

[afarber]

Fixes #13513

Refactored MemoryEndPointPipe to use RetainableByteBuffer.DynamicCapacity instead of allocating ByteBuffers directly.

Changes:

• Replace Deque<ByteBuffer> with Deque<RetainableByteBuffer>
• Each flush() creates a DynamicCapacity buffer via append() and adds it to the deque
• fillInto() reads from the deque using get(), releasing buffers when fully consumed
• Preserve existing EOF sentinel pattern for proper data/EOF ordering
• Add edge case tests for EOF handling, close behavior, and capacity limits
• Add notes to migration docs