fix: add default timeout for terminal command tool execution (#10550)

* fix: add default timeout for terminal command tool execution

Terminal commands with waitForCompletion=true had no timeout, risking
indefinite hangs. Adds a 2-minute default timeout with graceful
SIGTERM -> 5s grace period -> SIGKILL escalation. Both streaming
and non-streaming code paths are covered. The SIGKILL timer is
properly cleared if the process exits during the grace period.

* fix: correct process running check and clear SIGKILL timer

Replace `childProc.killed` with `exitCode === null && signalCode === null`
for accurate process-state detection, and clear sigkillTimeoutId in error
handlers to prevent delayed kills after rejection.

Applied to both streaming and non-streaming paths.

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Co-authored-by: amabito <192487536+amabito@users.noreply.github.com>
Co-authored-by: amabito <amabito@local>