Flaky test KillingLimit_TerminatesOverCapProcess makes CI on main red (child exits before the kill) - #762

…erministic

Make termination by the output limit the only way the child process ends in
KillingLimit_TerminatesOverCapProcess by using an unbounded producer
('exec yes 0123456789abcdef') and a bounded CancellationToken safety net.
Assert StdoutLimitExceeded, !Success, and that the process terminated with a
non-zero exit code (SIGKILL exit code 137 on Linux).

Add a separate deterministic test
KillingLimit_CleanExitBeforeKill_ReportsSuccessWithLimitExceeded documenting
and asserting that if a process finishes and exits cleanly with code 0 before
the runner's limit-kill takes effect, the runner faithfully reports ExitCode = 0,
Success = true, and StdoutLimitExceeded = true with bounded stdout retention.

CodeyBox-Prompt-Revision: 1
Co-Authored-By: CodeyBox <noreply@codeybox.invalid>