sql: generator builtin close() method does not surface errors

[angelapwen]

Describe the problem
In creating a SQL generator method, we use the tree.ValueGenerator interface. Its Close() method currently does not have a return value.

This poses a problem when using the new Internal Executor's streaming iterator pattern (added in #59330) to generate builtin function. Its Close() method must be called to close the iterator and returns an error, but this error cannot be surfaced in the generator builtin caller (and the iterator may potentially be left open).

This problem was originally discussed in context of #60922: link

Jira issue: CRDB-3064

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