SNOW-3489991: Block flattening of dropped NEW columns in connect mode

[sfc-gh-joshi]

In can_select_statement_be_flattened, the existing guard only refused to flatten when a dropped NEW column was also is_referenced_by_same_level_column. This missed the case where a future filter/sort (not yet added at select() time) references the dropped column, generating invalid SQL.

In connect mode, extend the guard to block flattening unconditionally when a NEW column is dropped. This preserves the defining subquery and keeps the column visible to subsequent WHERE/ORDER BY clauses. Non-connect behavior is unchanged.

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-3489991

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3. Please describe how your code solves the related issue.

   Please write a short description of how your code change solves the related issue.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.49%. Comparing base (9975596) to head (b1a9a74).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4259   +/-   ##
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  Coverage   95.49%   95.49%           
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  Files         171      171           
  Lines       44243    44244    +1     
  Branches     7548     7548           
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+ Hits        42252    42253    +1     
  Misses       1226     1226           
  Partials      765      765           

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