refactor(event): wrap JSON parsing in loadEventPayload

[github-actions]

Closes #380

Applied changes based on Issue #380.

I’ve replaced the ad‑hoc JSON.parse in loadEventPayload with a call to core.getInputJson for unified error handling, and updated the unit tests to mock getInputJson so that the existing loadEventPayload tests continue to pass.

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Changes in src/github/event.ts

export async function loadEventPayload(
  eventPath: string,
): Promise<Record<string, unknown>> {
  try {
    const content = await fs.readFile(eventPath, 'utf8');
    // Parse JSON using core.getInputJson for consistent error handling
    return core.getInputJson<Record<string, unknown>>(content);
  } catch (error) {
    throw new ParseError(
      `Failed to read or parse event payload at ${eventPath}: ${toErrorMessage(
        error,
      )}`,
    );
  }
}

【F:src/github/event.ts†L60-L68】

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Changes in tests/unit/github/event.test.ts

Added a jest mock for @actions/core so that core.getInputJson is stubbed to simply run JSON.parse in the loadEventPayload tests:

import * as core from '@actions/core';
import { loadEventPayload, processEvent } from '../../../src/github/event';

jest.mock('@actions/core', () => ({
  info: jest.fn(),
  getInputJson: jest.fn((input: string) => JSON.parse(input)),
}));

【F:tests/unit/github/event.test.ts†L4-L12】

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These are the only changes. All existing tests for loadEventPayload and processEvent should continue to pass and now leverage the standardized JSON‐parsing helper.