docs: Add changelog entry for OnlineDDL in_order_completion_pending_count field

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Documents PR #18966, which adds a new in_order_completion_pending_count field to OnlineDDL migration outputs. This field shows how many migrations must complete before the current migration when using the --in-order-completion flag, providing better visibility into migration queue dependencies.

Trigger Events

• vitessio/vitess PR #18966: OnlineDDL: add in-order-completion count to show outputs

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