refactor: Enhance token exchange logic for RFC8693 compliance

[yuito-it]

Summary

This PR improves the needRFC8693TokenExchange function in the Pinniped CLI to handle both string and slice types ([]string / []interface{}) for the aud claim in ID tokens when checking against requestedAudience.

Background / Details

• Previously, when an IdP returned the aud claim as an array/slice (e.g., ["client-id"]), comparing it directly against a string requestedAudience caused type mismatches or unexpected condition evaluations.
• This change ensures that the CLI safely checks whether requestedAudience is present in the aud claim regardless of whether aud is formatted as a single string or a slice of strings/interfaces.

Fixes

none

Release note

Fixed an issue in Pinniped CLI where RFC8693 Token Exchange determination logic failed to properly evaluate ID token `aud` claims when formatted as an array/list.

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