fix bug when rewriting sql virtual column registry

[clintropolis]

Description

Fixes a bug in #12241 from a simple mistake I made that results in modifying the Map that is being iterated over ending up in a ConcurrentModificationException. The added test fails to plan due to this error prior to the changes in this PR, with one of these in the stack trace:

Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1469)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1503)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1501)
	at org.apache.druid.sql.calcite.rel.VirtualColumnRegistry.visitAllSubExpressions(VirtualColumnRegistry.java:212)
	at org.apache.druid.sql.calcite.rel.DruidQuery.getVirtualColumns(DruidQuery.java:643)

I tried to fix this by using replaceAll, but still saw the exception, so ended up just copying everything to a new collection and going through them that way.

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This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[cheddar]

Making the check mark green.