Summary:
Original commit: b3c9d83ae0fe250b65bbfbd36a35ff0fdf5606d6 / D54051
There may be be legitimate cases when a merge key does not have a
condition. Optimizer can skip the condition if they are constantly
true in a context of other condition. Simple example if the same
column occurs multiple times in the index, like in:

`CREATE INDEX ON t(a, b, a, c);`

Query may have conditions on a and b and require order by c.
The executor can simply ignore the second a and merge sort by c.

So this diff removes assertions implying that all the stream key
columns must have a condition on them.

Backport-through: 2025.2

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressMergeScan'

Reviewers: aagrawal, jason

Reviewed By: jason

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D54118