Share byte buffers across exec-server output chunks (#49972)

## Why

Retaining process output, publishing events, and serving reads cloned the underlying byte buffers. Sharing those buffers avoids copying output bytes when chunks are cloned.

## What changed

Store `ByteChunk` bytes in `Arc<Vec<u8>>` and retain `ProcessOutputChunk` values directly. Add borrowed byte access for the MCP process transport while preserving base64 serialization and owned byte extraction through `into_inner()`.

## Testing

Add a unit test covering base64 serialization, deserialization, and independent owned bytes extracted from shared chunks.

GitOrigin-RevId: 6bba114af48424aa844487aa75163cfc604e0575