fix(core): gate unified exec output streaming

[joshka-oai]

fix(core): gate unified exec output streaming

Context

CI for openai/codex#7601 exposed two issues with unified exec output collection:

• suite::shell_snapshot::linux_unified_exec_uses_shell_snapshot failed on ubuntu‑24.04‑arm with stdout="\n" instead of the expected snapshot-linux marker. This indicates we sometimes capture only a partial/whitespace chunk from short‑lived commands before the remaining stdout arrives.
• suite::unified_exec::unified_exec_respects_early_exit_notifications timed out waiting for TaskComplete because the collection loop waited the full yield_time_ms after an early exit, stalling silent commands.

What’s changing

• Only start ExecCommandOutputDelta streaming after the session is confirmed alive, so early‑exit commands don’t emit delta events.
• When a process exits quickly, wait a short grace window for initial stdout so short‑lived commands (like the shell snapshot) can flush output.
• Treat whitespace‑only output as “still empty” so a lone newline doesn’t prevent the longer grace window on slower/ARM runners.

Why this is correct

These changes balance two competing behaviors:

• Capture meaningful output from fast‑exiting commands (e.g., snapshot generation).
• Avoid stalling turn completion when commands are silent (which caused the TaskComplete timeout).

The grace window is short and only applied after exit, preserving responsiveness while making output capture more robust.

Tests/CI signal

• Previously failing:
  • suite::shell_snapshot::linux_unified_exec_uses_shell_snapshot (stdout only "\n" on ubuntu‑24.04‑arm)
  • suite::unified_exec::unified_exec_respects_early_exit_notifications (timeout waiting for TaskComplete)
• With this change, the viewport PR’s CI went green, indicating these failure modes are resolved.

Files touched

• core/src/unified_exec/session_manager.rs