Add APIError class to preserve structured error data from handleResponse

[Copilot]

handleResponse was serializing the structured API error object (with status, code, message fields) into a plain Error message string, forcing consumers to re-parse JSON to distinguish error types.

Changes

• New APIError class — exported, extends Error, carries status: number and code: string directly on the instance
• Updated handleResponse — detects structured error shape (status/code/message) and throws APIError; falls back to plain Error for unstructured errors (preserving existing behavior)
• Tests — added coverage for APIError construction, and structured 401/404/429 responses asserting correct instanceof, status, and code

Usage

try {
  await client.evm.tokens.getHolders({ ... });
} catch (e) {
  if (e instanceof APIError) {
    if (e.status === 429) { /* retry */ }
    if (e.status === 404) { /* not found */ }
  } else {
    // Network error — TypeError/Error from fetch (DNS, socket, timeout)
  }
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>handleResponse loses structured error data by stringifying into Error message</issue_title>
<issue_description>handleResponse currently does:

token-api-sdk/src/index.ts

Lines 226 to 234 in 62cfac5

	function handleResponse<T>(data: T | undefined | null, error: unknown): T {
	if (error) {
	throw new Error(`API Error: ${JSON.stringify(error)}`);
	}
	if (data === undefined || data === null) {
	throw new Error('API Error: No data returned');
	}
	return data;
	}

The error object from openapi-fetch is the parsed JSON error body with structured fields (status, code, message), but this gets flattened into a string. Consumers who need to distinguish error types (e.g., retry on 429 but not on 404) have to parse JSON back out of the error message.

Suggestion

Throw a typed APIError that preserves the structured data:

export class APIError extends Error {
    status: number;
    code: string;

    constructor(error: { status: number; code: string; message: string }) {
        super(error.message);
        this.name = 'APIError';
        this.status = error.status;
        this.code = error.code;
    }
}

A single error class is enough — the SDK only catches API-level errors in handleResponse. Network errors (DNS, socket, timeout) from fetch already propagate as native TypeError/Error and don't go through handleResponse. Consumers can distinguish them with instanceof APIError.

try {
    await client.evm.tokens.getHolders({ ... });
} catch (e) {
    if (e instanceof APIError) {
        // API error — e.status, e.code available
        if (e.status === 429) { /* retry */ }
        if (e.status === 404) { /* not found */ }
    } else {
        // Network error — DNS, socket, timeout
    }
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes handleResponse loses structured error data by stringifying into Error message #50

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