fix: contested unique indexes can only be on non mutable document types

[QuantumExplorer]

Issue being fixed or feature implemented

Extra validation that there can only be one contested unique index, and that we can only have a contested unique index when the document type is non mutable.

What was done?

How Has This Been Tested?

Breaking Changes

Checklist:

• I have performed a self-review of my own code
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• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

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