Fix crash in Puma validation initializer

What does this MR do?

Fixes a boot-time crash introduced in !51906 (merged) due to a method ordering problem. (allow_single_mode? was called before it was defined.)

The unit test loads this initializer; I am not sure why it did not detect this issue.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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