Proto: migrate trivial single-child plans (CoalesceBatches, CoalescePartitions, Cooperative, Buffer)

[adriangb]

Part of #23494.

coalesce_batches.rs, coalesce_partitions.rs (proto variant Merge), coop.rs, buffer.rs. Each: single child + a scalar field or two. CoalesceBatchesExec is #[deprecated] — gate accordingly.

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Follow the pattern in #23494 (reference impl: #23495 / ProjectionExec; full end-state draft: #23496). Add try_to_proto inside impl ExecutionPlan for X, add the inherent X::try_from_proto, repoint the decode arm, and delete the old central arm in the same PR (that is the proof the hook is reached). Keep the wire format byte-for-byte identical.