Currency search using wrong encoding after hex revert

[linconvidal]

Context & versions

---

Currency search was added in v1.3.2. After reverting to hex encoding (#609), the /search/transactions endpoint needs to handle currency filtering with hex values, not ASCII.

Steps to reproduce

1. call /search/transactions with currency filter:

curl -H "Content-Type: application/json" \
    -d '{
      "network_identifier": {"blockchain": "cardano", "network": "mainnet"},
      "currency": {
        "symbol": "000de1404469616d6f6e64486f6f76657332363938",
        "decimals": 0
      },
      "limit": 5
    }' \
    'http://localhost:8082/search/transactions'

Actual behavior

The search endpoint in v1.3.2 expects ASCII values for currency.symbol field and may not correctly handle hex values, especially those with CIP-67 prefixes.

Expected behavior

The search should accept and correctly filter by hex-encoded currency symbols, including CIP-68 assets with their binary prefixes (e.g., 000de140 for label 222 NFTs).