[Gen4] Merge SeenPredicates when creating route operator for join

[arthurschreiber]

Description

In the Gen4 planner, Route structs hold a list of predicates that can influence the routing decisions (SeenPredicates). When merging inner queries into their outer query, vindex predicates are reset and their options are rebuild from this list of seen predicates.

Unfortunately, when a route object was build for an ApplyJoin operator, the lists of SeenPredicates from the left and right parts of the join were not copied over into the new route object. If a subquery was then merged into this Route, the vindex predicate options could not be rebuild and the resulting route would become a Scatter.

By merging the SeenPredicates lists of both sides of the join, we can ensure that the vindex predicate reset actually works correctly, and the correct vindex is used for routing.

Related Issue(s)

This is an alternative (and arguably simpler) fix to #11072.
This fixes #10823.

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[vitess-bot]

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