fix(tax): skip wu_get_country_states() for wildcard '*' country rates

[superdav42]

Summary

Completes the universal tax fallback feature (issue #511) by fixing the one missing piece: when a tax rate has country='*' (Apply to all countries), wu_get_country_states('*') was being called unnecessarily. This guard ensures wildcard rates receive an empty state_options array instead.

What was already in place on main

The bulk of the feature was already implemented across three prior PRs/branches:

• wu_get_applicable_tax_rates() (inc/functions/tax.php) — already returns * rates as a catch-all fallback when no country-specific rate matches the customer's country. Country-specific rates always take precedence.
• Tax Rates UI (views/taxes/list.php) — the country dropdown already includes <option value="*">Apply to all countries</option> as the first option.
• Unit tests (tests/WP_Ultimo/Functions/Tax_Functions_Test.php) — comprehensive tests for wildcard fallback behaviour already exist and pass.

What this PR adds

inc/tax/class-tax.php — get_tax_rates() method:

// Before
$rate['state_options'] = wu_get_country_states($rate['country'], 'slug', 'name');

// After
$rate['state_options'] = ('*' !== $rate['country'])
    ? wu_get_country_states($rate['country'], 'slug', 'name')
    : [];

When $fetch_state_options = true (called from the AJAX handler that serves the Tax Rates admin page), passing '*' to wu_get_country_states() would attempt to instantiate Country_* (which doesn't exist), fall back to Country_Default, and return an empty/meaningless result anyway. The guard makes this explicit and avoids the unnecessary lookup.

Testing

• PHP syntax: php -l passes on all three tax files
• Static review: diff is 8 lines, single-purpose, no logic change to the fallback mechanism
• Unit tests: 6 wildcard-specific tests in Tax_Functions_Test.php cover the fallback behaviour end-to-end — CI will run these

References

• Closes feat(tax): universal tax fallback — redesigned implementation (apply to all countries option) #511
• Original implementation context: PR feat(tax): add universal tax fallback feature #277 (closed, branch feature/universal-tax-rate)
• Maintainer spec: redesign as "Apply to all countries" option in the country dropdown, not a separate settings toggle

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📥 Commits

Reviewing files that changed from the base of the PR and between 87e9e46 and b64b66c.

📒 Files selected for processing (1)

• inc/tax/class-tax.php

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/t511-universal-tax-fallback

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[github-actions]

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[superdav42]

Note: WP Performance Metrics check failed on this PR. This is not a required check (required: PHP 8.2-8.5, Cypress), so it does not block merge. However, the tax fallback change may have introduced a performance regression — maintainer should review the performance results before merging.