fix(core): report shell exit only after output is captured - #56321

Problem

Shell.finish() set the terminal status (exited / timeout) before awaiting the output pump. During that window, GET /api/shell/:id already reported exited while the output file was still empty or partial, so a client that polls get and then reads output could get nothing back.

Reproduced over HTTP against the in-process server: polling shell.get until non-running, then reading shell.output for printf hello-world, returned "" in 10 of 40 runs.

Fix

Split "how did it end" from "publish that it ended":

• An ended Deferred records the outcome. The process exit and the timeout timer both complete it, and the first one wins. The timer records the timeout and starts the kill in one step that cannot be interrupted. The kill itself runs detached, so remove and teardown never wait on a dying process.
• One fiber, after the exit watcher, awaits ended, then waits for captured output to drain, and only then updates info, resolves waiters, publishes shell.exited, and runs retention.
• time.completed is still the moment the process ended or the timeout fired.

Behavior change: when a timed-out process ignores SIGTERM, get and list keep reporting running until it is force-killed (up to about 3s), and retiming is ignored during that window. Previously status switched to timeout as soon as the timer fired, even though the process was still alive and its output incomplete.

Verification

• Local scratch tests, not committed:
  • With a deterministic fake process that exits while its last output chunk is held back, get stays running until the chunk arrives. The same check fails on dev.
  • Retiming after a timeout still resolves wait with timeout.
• HTTP repro: 10/40 empty reads before, 0/40 after. A timeout: 200 command reports timeout after about 220ms. With a process that ignores SIGTERM, time.completed matches when the timeout fired, and remove during the kill returns in a few ms.
• Core shell tests: same results as dev. The 7 bash-syntax failures on macOS also fail on dev.
• bun typecheck in packages/core is clean.