[release-23.0] Fix cross shard/keyspace joins with derived tables containing a UNION. (#19046)

[vitess-bot]

Description

This is a backport of #19046

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.72%. Comparing base (ba2028e) to head (84406b0).
⚠️ Report is 1 commits behind head on release-23.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-23.0   #19137      +/-   ##
================================================
- Coverage         69.72%   69.72%   -0.01%     
================================================
  Files              1606     1606              
  Lines            214601   214604       +3     
================================================
+ Hits             149625   149627       +2     
- Misses            64976    64977       +1     

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