* feat(cron): add global session_mode config for cron jobs
Allow setting a default session_mode ("reuse" or "new_per_run") for all
cron jobs via [cron] config, so users don't need to set it per-job.
Per-job session_mode still overrides the global default.
* test(cron): add unit tests for global session_mode default
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Co-authored-by: ma_k <ma_k@ctrip.com>