## Why
Unified exec can drain process output multiple times while waiting for a command. Collecting those drains into an uncapped buffer allows large commands to exceed the output collection limit.
## What changed
- Accumulate drained output through the capped head/tail buffer so collection remains bounded while preserving the beginning and end of the stream.
- Include a `... N bytes omitted ...` marker when the collection cap drops output.
- Calculate the original token estimate from all observed bytes and preserve omission metadata through truncation and sandbox-denial responses.
## Testing
- Cover repeated drains, previously omitted output, omission markers, and large-output summaries.
GitOrigin-RevId: c6e2efeb875ed8f35914365eab7431f25fd3484c
6138909d6e