Migrate from @octokit/action to @octokit/core for better maintainability

[Copilot]

Overview

This PR migrates the GitHub API client from @octokit/action to @octokit/core to improve maintainability and follow official Octokit recommendations. The @octokit/action package is designed specifically for direct JavaScript usage in GitHub Actions workflows, while @octokit/core provides a more general and future-proof approach.

Changes Made

Dependencies

• Removed: @octokit/action@^6.0.7
• Added: @octokit/core@^7.0.2 and @octokit/plugin-rest-endpoint-methods@^16.0.0

Code Changes

• Updated src/install.ts to use @octokit/core with explicit GITHUB_TOKEN authentication
• Enhanced error handling to throw a clear error message when GITHUB_TOKEN is missing
• Modified the findLatestVersion() function to explicitly check for and require GITHUB_TOKEN

Testing Improvements

• Added Jest mocks to handle ES module compatibility with the new Octokit packages
• Enhanced test coverage with a new test case for missing GITHUB_TOKEN scenario
• Updated Jest configuration to properly handle the new dependencies

Configuration Updates

• Updated ESLint configuration to handle the new package imports
• Minor README update to use current GitHub API documentation links

Behavior Changes

Before

// Automatic token handling - would fail silently in some cases
const octokit = new Octokit()
const response = await octokit.repos.getLatestRelease({...})

After

// Explicit token handling with clear error messages
const token = process.env.GITHUB_TOKEN
if (!token) {
  throw new Error('GITHUB_TOKEN environment variable is required to fetch the latest version')
}
const MyOctokit = Octokit.plugin(restEndpointMethods)
const octokit = new MyOctokit({ auth: token })
const response = await octokit.rest.repos.getLatestRelease({...})

Backward Compatibility

✅ Fully backward compatible - No breaking changes to the public API. Users will continue to use the action exactly as before, but will now receive clearer error messages when GITHUB_TOKEN is missing.

Testing

All existing tests pass, plus new test coverage for:

• ✅ Latest version installation with GITHUB_TOKEN provided
• ✅ Clear error message when GITHUB_TOKEN is missing
• ✅ All existing version-specific installations continue to work

Benefits

1. Better maintainability: Using the officially recommended Octokit approach
2. Clearer error messages: Users now get explicit feedback when GITHUB_TOKEN is missing
3. Future-proof: Easier to update and maintain with future Octokit releases
4. Better testing: Improved test coverage with proper mocking strategy

The migration ensures that the action continues to work exactly as expected while providing a more robust foundation for future development.

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[KengoTODA]

@copilot why this change is necessary for this PR?

[Copilot]

The ESLint rule change is necessary because the new @octokit/core and @octokit/plugin-rest-endpoint-methods packages are ES modules (they have "type": "module" in their package.json), which causes ESLint's import/no-unresolved rule to fail to resolve their imports. Without this ignore rule, ESLint throws errors like "Unable to resolve path to module '@octokit/core'". The rule specifically ignores these two packages while maintaining import resolution checking for all other dependencies. Commit: 53879e5