refactor(utils): simplify error utility module

[github-actions]

Closes #551

Applied changes based on Issue #551.

I’ve refactored src/utils/error.ts to centralize the instanceof Error check into a reusable type‐guard, and updated the two existing helpers to leverage it—improving readability and avoiding duplication, with zero change in behavior. Here’s what was done:

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Changes in src/utils/error.ts

/**
 * @file Error handling utilities module.
 * Provides functions to convert unknown errors safely to message and stack strings.
 */

/**
 * Type guard to determine if a value is an Error.
 * @param error - The value to check.
 * @returns True if the value is an instance of Error.
 */
function isError(error: unknown): error is Error {
  return error instanceof Error;
}

/**
 * Safely extracts the message from an unknown error object.
 * @param error - The caught error or value.
 * @returns The error message string.
 */
export function toErrorMessage(error: unknown): string {
  return isError(error) ? error.message : String(error);
}

/**
 * Safely extracts the stack trace from an unknown error object.
 * @param error - The caught error or value.
 * @returns The error stack trace or undefined if unavailable.
 */
export function toErrorStack(error: unknown): string | undefined {
  return isError(error) ? error.stack : undefined;
}

【F:src/utils/error.ts†L6-L13】【F:src/utils/error.ts†L20-L22】【F:src/utils/error.ts†L29-L31】

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Summary of refactoring:

• Extracted a private isError() type‐guard for checking instanceof Error.
• Consolidated the duplicate instanceof logic in toErrorMessage and toErrorStack into a single call to isError.
• Kept all existing JSDoc comments and overall functionality identical.

No other files were touched. Let me know if you’d like any additional adjustments!