test: add integration and unit tests with fixtures

[parkerbxyz]

Introduces comprehensive integration and unit tests using Node.js built-in test runner. Adds before/after fixtures, snapshot testing, and a test README. Refactors diff generation logic for consistency and updates scripts in package.json to support testing and coverage.

[Copilot]

Pull Request Overview

This PR introduces comprehensive testing infrastructure for the suggest-changes action, including both unit and integration tests using Node.js built-in test runner. The changes also refactor the diff generation logic for better consistency and modularity.

Key changes:

• Adds unit tests for core functions with mock dependencies and edge case coverage
• Implements integration tests with snapshot testing using fixture-based before/after file pairs
• Refactors git diff generation into a shared utility function for consistency

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/unit.test.js	Unit tests for generateCommentKey, createSuggestion, and run functions with mocked dependencies
test/integration.test.js	Integration tests that discover fixture pairs and verify suggestion generation with snapshots
test/integration.test.js.snapshot	Snapshot file containing expected output for markdownlint fixture tests
test/fixtures/markdownlint/before.md	Test fixture with various markdown formatting issues for testing
test/fixtures/markdownlint/after.md	Corrected version of markdown fixture showing expected formatting
test/fixtures/markdownlint/README.md	Documentation explaining the markdownlint test fixtures and their purpose
test/README.md	Comprehensive documentation for the testing setup, structure, and usage
package.json	Adds test scripts and Node.js engine requirement for testing capabilities
index.js	Refactors diff generation into shared getGitDiff function and updates parameter naming
.github/workflows/test.yml	GitHub Actions workflow for running tests on pull requests

Comments suppressed due to low confidence (2)

test/unit.test.js:99

• The test uses a minimal mock that only implements listReviewComments but doesn't test the case where createReview would be called. Consider adding a test that verifies the review creation path when comments are generated.

      const result = await run({
        // @ts-ignore - Test mock doesn't need full Octokit interface
        octokit: mockOctokit,

test/unit.test.js:133

• This test expects reviewCreated to be true and comments to be generated, but the mock diff provided may not actually generate any comments through the real parsing logic. Consider using a diff that you've verified produces suggestions or mock the generateReviewComments function.

      const result = await run({
        // @ts-ignore - Test mock doesn't need full Octokit interface
        octokit: mockOctokit,