Add LibraryInitializer registration tests

[nodh]

Motivation

• Verify that LibraryInitializer.registerExtensionLibrary correctly registers credential schemes and polymorphic serializers for CredentialSubject subtypes.
• Ensure ISO-MDOC registration overload wires jsonValueEncoder and CborCredentialSerializer so values can be encoded/decoded in IssuerSignedItem instances.

Description

• Add src/commonTest/kotlin/at/asitplus/wallet/lib/LibraryInitializerTest.kt containing tests that register a dummy CredentialScheme and a SerializersModule with a test CredentialSubject subclass.
• Create a TestCredentialScheme implementation and validate registration via AttributeIndex.resolveAttributeType(...) for the vcType lookup.
• Add an ISO-focused test that provides a jsonValueEncoder and itemValueSerializerMap, asserts JsonCredentialSerializer.encode(...) behavior, and performs a CBOR round-trip using IssuerSignedItemSerializer and coseCompliantSerializer.

Testing

• No automated tests were executed as part of this change.
• A new unit test file LibraryInitializerTest.kt was added that exercises LibraryInitializer.registerExtensionLibrary and ISO encoder/serializer round-trip behavior, but it has not been run in CI for this PR.

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Codex Task

[JesusMcCloud]

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