Show correct variations count when generating a single variation

[mattsherman]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

This PR corrects the variations count shown when using the "Generate variations" button and a single variation is generated.

Before

After

Closes #37644.

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Go to Products > Add New > Product data.
2. Select a Variable product and go to the Attributes tab.
3. Add a new attribute with only one value/term and save the attribute.
4. Go to the Variations tab and click Generate variations.
5. Verify that the variations count is correct.
6. Also, use Add manually and Remove to add variations and verify the count remains correct.
7. Repeat this with an attribute with more than one value/term and verify that when multiple variations are generated, the count is correct.

[github-actions]

Hi @octaedro,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[github-actions]

Test Results Summary

Commit SHA: a3eee81

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	267	0	0	2	0	269	1m 15s
E2E Tests	187	0	0	10	0	197	14m 10s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[octaedro]

Nice job @mattsherman. The changes look good, I only left a couple of comments.
Maybe I have a building problem, but I'm seeing the confirmation window like this:

[mattsherman]

Nice job @mattsherman. The changes look good, I only left a couple of comments. Maybe I have a building problem, but I'm seeing the confirmation window like this:

This happens when the legacy JS isn't built. I just hit it when forgetting to pnpm run build after switching back to this branch.

[mattsherman]

@octaedro Ready for re-review!

[octaedro]

Great work @mattsherman! LGTM