Port and extend the agent environment tests for NodeExecutionEnv, output
capture, adaptive publishing, and truncation, including exact-byte
truncation, flushing, line-reader cancellation, spill backpressure, and
Buffer-free UTF-8 counting.
Fix truncation reporting when bytes and lines are both exceeded, when only
bytes are exceeded at the line cap, and when small limits trim multi-byte
edge characters. Report the full size of oversized newline-terminated tail
lines. Resolve durable test imports from source so tests run unbuilt.
Set Pico tool output defaults to 50 KiB and 2,000 lines.
