fix: decode wrapped LINE token refresh responses

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Issues

All clear! No issues remaining. 🎉

2 issues already resolved

• A refresh response with a valid top-level token and "data": null now fails. json.RawMessage holds the bytes null, so response.Data != nil is true and res is reset to an empty struct, which gives an empty AccessToken. (fixed by commit 2a02eaa)
  _{Found by Indent Review Agent}
• Classified refresh failures now carry a reason, but unclassified ones still return only refresh rejected: code N or refresh response missing access token. An unexpected LINE error code or unrecognized 200 body leaves nothing in the logs to diagnose it; the old error included the body. (fixed by commit 7cc6978)
  _{Found by Indent Review Agent}

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Passed (1)

• Indent Review Agent · No new issues: earlier nit resolved; new diagnostic is panic-safe, leaks no secrets.

Full results

Indent Review Agent

• Summary: No new issues: earlier nit resolved; new diagnostic is panic-safe, leaks no secrets.
• Last ran on commit: 7cc69784
• Latest result

  {
    "summary": "No new issues: earlier nit resolved; new diagnostic is panic-safe, leaks no secrets.",
    "findings": []
  }

[linear-code]

PLAT-39178