/v1/nfts/balance/{ownerAddress} returns contract_address instead of token_address

[masahirodev]

Description

Currently, the API returns contract_address, while the documentation specifies token_address.
Since existing clients may already rely on token_address, removing or renaming it could cause breaking changes.

To maintain backward compatibility and improve clarity,
we propose to include both fields in the response, and mark token_address as deprecated in the documentation.

Expected Response

{
  "token_address": "0x1234...abcd",      // deprecated
  "contract_address": "0x1234...abcd"
}