refactor: address claims accessor review follow-ups

[SeanROlszewski]

Follow-up to the four review comments on #490:

1. Trim the claims() doc comment — reduced to the summary line.
2. Arc in claims() -> Vec<Arc<FieldElement>> — investigated and it's load-bearing: uniffi objects only implement Lower behind an Arc, so sequences of objects must be Vec<Arc<T>>. Dropping it fails scaffolding compilation with the trait bound 'FieldElement: uniffi::Lower<UniFfiTag>' is not satisfied (bare owned objects are accepted only as top-level return values, which is why sub() gets away without one). Left a code comment so the next reader doesn't retry it.
3. Remove claims_hex() — gone; callers now read claims() and encode the specific elements they need, making per-claim selection the deliberate act.
4. Trim the fetch_credential doc comment — reduced to the summary + # Errors (kept for missing_errors_doc).

Testing: cargo test -p walletkit-core --lib green (tests updated for the removed method); clippy (pedantic) clean.

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Note

Low Risk
Small public API removal on FFI Credential; callers must switch to claims() + per-element encoding, with no auth or storage logic changes.

Overview
Follow-up cleanup on the credential claims API: claims_hex() is removed, so callers must use claims() and encode individual FieldElement values (e.g. to_hex_string()) when they need hex.

claims() and fetch_credential doc comments are shortened; unit and storage tests are updated to assert via claims() instead of the removed helper.

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