feat(tax): universal tax fallback — redesigned implementation (apply to all countries option)

[superdav42]

Background

PR #277 by @mahyarrezghi implemented a universal tax fallback feature but was closed in favour of a redesigned approach.

Requested Implementation

Per maintainer review on PR #277, the universal/fallback tax should be implemented as:

• An 'Apply to all countries' option in the country-specific tax rate configuration UI
• Rather than a separate toggle + rate field in general settings

Original PR

PR #277 added:

• enable_universal_tax setting toggle
• universal_tax_rate setting (0-100%)
• apply_universal_tax_rate filter
• Translation strings

The redesigned version should achieve the same outcome (fallback rate when no country-specific rate matches) but integrated into the existing country tax rate UI.

References

• Closed PR: feat(tax): add universal tax fallback feature #277
• Original contributor: @mahyarrezghi

[superdav42]

Dispatching worker.

• Model: sonnet (anthropic/claude-sonnet-4-6)
• Branch: feat/t511-universal-tax-fallback
• Scope: Implement universal tax fallback as 'Apply to all countries' option per maintainer spec in PR feat(tax): add universal tax fallback feature #277 review
• Attempt: 1 of 1
• Direction: Reference closed PR feat(tax): add universal tax fallback feature #277 (branch exists) for context on the original implementation. The redesign should add an 'Apply to all countries' checkbox/option to the tax settings.

[superdav42]

Completed via PR #512

What was done:

• Fixed the one missing piece of the universal tax fallback feature: class-tax.php get_tax_rates() now guards against calling wu_get_country_states('*') when a rate has country='*' (Apply to all countries). Wildcard rates now correctly receive an empty state_options array.
• The rest of the feature was already in place on main: the wu_get_applicable_tax_rates() fallback logic, the "Apply to all countries" option in the Tax Rates UI dropdown, and comprehensive unit tests.

Testing Evidence:

• Testing level: self-assessed (PHP syntax check passed; unit tests run in CI)
• Stability results: PHP 8.2/8.3/8.4/8.5 all pass; Cypress E2E passes; CodeRabbit review passed; WP Performance Metrics failure is a pre-existing flaky test (server startup timeout, unrelated to this change)
• Smoke pages/endpoints checked: N/A — change is a guard clause in a PHP method, no UI change

Key decisions:

• Used a ternary guard ('*' !== $rate['country'] ? wu_get_country_states(...) : []) rather than an early return to keep the change minimal and readable
• Did not add a new settings toggle — per maintainer spec in PR feat(tax): add universal tax fallback feature #277 review, the feature is integrated into the existing country dropdown as an option, not a separate settings field

Files changed:

• inc/tax/class-tax.php — guard wu_get_country_states() call when country='*'

Blockers encountered: None

Follow-up needs: None — the feature is complete. Admins can now create a tax rate with "Apply to all countries" selected in the country dropdown, and it will act as a catch-all fallback rate for any customer whose country has no specific rate configured.