Fix search bug in prompt related to custom item types

[patriksvensson]

Fixes #1626

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

ListPromptState<T> now takes converter into account.

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