refactor: extract requireValidAddress/requireValidToken helpers

[TimNooren]

Summary

• Extracted two small helpers (requireValidAddress, requireValidToken) in api.js that replace 21 identical validate-and-throw blocks
• Each 3–4 line block (validate, check result, throw) becomes a single function call
• Net: −43 lines (37 added, 80 removed), single file change

Test plan

• npm test — all 1281 tests pass
• npm run lint — clean
• No behavior change — helpers throw the exact same NansenError with the same error/code

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