Ensure token item and metadata keychain accessibility settings are consistent.

[mikenachbaur-okta]

This addresses #198 by:

1. Ensuring the token metadata is stored with the appropriate accessibility settings, respecting the parent item's iCloud sync setting.
2. Returning token IDs only if both their item and metadata keychain entries can be found.
3. Establishing the Credential.Security.isDefaultSynchronizable property, allowing the default identifier to be shared.

This also adds unit test coverage to validate these changes.