Improve migration flow to new OAuth authentication for a better user experience.

[iamdharmesh]

Description of the Change

This PR improves the user experience for migrating from API key authentication to the new OAuth authentication. The existing flow requires a two-step process (logout and login with OAuth authentication). This new flow introduces a "Migrate to OAuth authentication" button in the notice on the settings page, making the process a single-step, easier, and confusion-free.

How to test the Change

1. Install the old version of the plugin 1.5.8.
2. Configure the plugin by adding the API key and saving the settings.
3. Checkout this PR.
4. Verify that an admin notice to migrate to OAuth authentication is displayed in the dashboard.
5. Verify that an admin notice with the "Migrate to OAuth authentication" button is displayed on the plugin settings page.
6. Click the "Migrate to OAuth authentication" button and verify that it triggers the Mailchimp OAuth login flow.
7. Follow the steps in the opened modal and complete the process. You should be redirected to the plugin settings page, with the authentication successfully migrated to OAuth.
8. Verify that the settings remain unchanged.

Changelog Entry

Changed - Improved migration flow from API Key to OAuth authentication for a better user experience.

Credits

Props @iamdharmesh @jeffpaul

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[qasumitbagthariya]

QA Update ✅

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I have verified this PR in the enhancement/migrate-to-oauth branch which has been fixed and is functioning as intended.

The migration to OAuth authentication works as expected, with no changes to existing settings.

I tested the following on this branch:

• Checked the PR and verified that an admin notice to migrate to OAuth authentication is displayed in the dashboard.
• Confirmed that an admin notice with the "Migrate to OAuth authentication" button is present on the plugin settings page.
• Clicked the "Migrate to OAuth authentication" button, which successfully triggered the Mailchimp OAuth login flow.
• Followed the steps in the opened modal and completed the process.
• Verified that I was redirected to the plugin settings page with the authentication successfully migrated to OAuth.
• Confirmed that all settings remained unchanged after the migration.

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Testing Environment

Details

• WordPress: 6.6.2
• Theme: Storefront 4.6.0
• Theme: Twenty Twenty-Four 1.2
• PHP: 8.0.30
• Web Server: Nginx 1.20.2
• Browser: Chrome
• OS: macOS Ventura 13.3
• Branch: enhancement/migrate-to-oauth

Steps to Test- As mentioned in the PR description.
Test Results - It is working as expected.
Functional Demo / Screencast -
Special Notes - Ready for code review (Woo)
Testing Document status:
Cases related to this Issue/PR are added to the Critical Flow Wiki pages:

• Yes
• Not Required/Applicable for this PR

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[jeffpaul]

Looks good to me in testing, ok to move to Regression Testing if you're 👍🏼 @dkotter

[qasumitbagthariya]

Regression / Smoke Test Report ✅

I have verified this PR in the smoke-test branch and it is working as expected.

Testing Environment

Details

• WordPress: 6.6.2
• Theme: Twenty Twenty-Four 1.2
• WooCommerce - 9.3.3
• PHP: 8.0.30
• Web Server: Nginx 1.20.2
• Browser: Chrome
• OS: macOS Ventura 13.3
• Branch: smoke-testing

Next Step- Ready to Merge 🚀

[vikrampm1]

@dkotter this PR commits into fix/persist-list branch as opposed to develop so I would let you review and merge accordingly.