Avoid repeated CLI output string concatenation

[rohangpta]

Why

stream_cli_command keeps complete stdout/stderr for the fallback RuntimeResult while also buffering partial lines for parser callbacks. Long-running CLI agents can produce large streams, and repeated += copies the accumulated string on each chunk, adding avoidable CPU and memory churn.

This is scoped to the shared CLI-agent helper after runtime events have already been emitted; it does not change runtime stream processing.

What Changed

• Accumulate stdout/stderr chunks in lists and join only when draining parser buffers or building the fallback RuntimeResult.
• Share stdout/stderr line-buffer flushing through a small helper.
• Add a focused unit test for split stdout/stderr chunks and fallback output preservation.