we were not respecting turn's `truncation_policy` to clamp output tokens
for `unified_exec` and `write_stdin`.
this meant truncation was only being applied by `ContextManager` before
the output was stored in-memory (so it _was_ being truncated from
model-visible context), but the full output was persisted to rollout on
disk.
now we respect that `truncation_policy` and `ContextManager`-level
truncation remains a backup.
### Tests
added tests, tested locally.