Fix: Create a request cache to ensure that each credential is only fetched once

[erbenjak]

Readiness checklist

• I added/updated tests.
• I ensured that the PR title is good enough for the changelog.
• I labeled the PR.
• I self-reviewed the PR.

[Copilot]

Pull request overview

This PR implements an in-memory cache for credential offer requests to prevent duplicate API calls when the same credential offer URL appears multiple times in a single request expansion. The cache stores Promises to ensure concurrent calls for the same URL share the same underlying request, addressing a race condition in the existing persistent cache implementation.

Changes:

• Added an in-memory Promise-based cache (credentialOfferCache) to the DataViewExpander class
• Modified credential offer request handling to check the cache before making new requests
• Wrapped the API call in error handling that returns undefined on failure

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