Add tests for RemoteResourceRetrieverFunction behavior

[nodh]

Motivation

• Ensure implementations of RemoteResourceRetrieverFunction follow the documented behavior of returning the response body when present or the Location header when the server responds with a redirect.
• Cover higher-level consumers that rely on the retriever to fetch referenced resources during request parsing and credential offer retrieval.

Description

• Add a new test file vck-openid/src/commonTest/kotlin/at/asitplus/wallet/lib/RemoteResourceRetrieverFunctionTest.kt with unit tests exercising the retriever behavior.
• Implement FakeRemoteResourceRetriever and FakeHttpResponse in the test to simulate responses with a body or a Location value.
• Add a test asserting that a body payload is used by RequestParser when present (request_uri case) and a test asserting that a Location redirect is followed by WalletService when fetching a credential_offer_uri.
• Wire the fake retriever into RequestParser and WalletService instances inside the tests to validate the end-to-end behavior.

Testing

• Added unit tests under vck-openid/src/commonTest, specifically two tests validating body-first and location-redirect behavior.
• No automated test runs were executed as part of this change.

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Codex Task

[JesusMcCloud]

Seems legit