sql/schemachanger: Ensure column alterations check for dropped status

[spilchen]

In the Declarative Schema Changer (DSC), three operations—ALTER COLUMN TYPE, SET NULLABILITY, and SET DEFAULT—did not verify whether a column had already been marked for drop. This could lead to issues if a DROP COLUMN and one of these operations were executed within the same transaction.

This update ensures that when these operations look up a column element, they also check its target status to confirm it has not been dropped.

Epic: CRDB-25314 Closes: #134008 Release note: None

[cockroach-teamcity]

This change is 

[spilchen]

TFTR!

bors r+

[craig[bot]]

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