WOOTAX-25 - Add regression test for zero-amount tax calculations

[Abdalsalaam]

Description

When a subscription is switched, or when a free-trial subscription's initial cart total is $0, TaxJar returns a response whose top-level rate (and amount_to_collect) is 0 because taxable_amount is 0 — while the real per-jurisdiction rates are still present inside breakdown.line_items. Historically the rate builder wrote the zeroed top-level rate into the shared WooCommerce tax-rate row, which set an existing rate to 0.0000% (WOOTAX-18) and left subscription renewals with no tax (WOOTAX-25).

The current get_itemized_tax_rates() already persists the real per-jurisdiction rates from breakdown.line_items and never writes the zeroed top-level rate (fixed by the itemized-tax-rate refactor). This PR is a test-only change that locks that behaviour in with a regression test, so the zeroing-out cannot silently come back.

The test feeds the exact zero-amount response captured in WOOTAX-18 into get_itemized_tax_rates() and asserts the real per-jurisdiction rates (city 1.5%, county 1.25%, state 4.225%) are persisted and the combined rate is 6.975%, never 0. It was verified to fail if the builder is reverted to writing the top-level rate.

Related issue(s)

Linear: WOOTAX-25 (same root cause as WOOTAX-18 / #2826).

Steps to reproduce & screenshots/GIFs

Original bug (no longer reproducible on trunk):

1. Enable automated tax rates (WooCommerce Tax).
2. Create a subscription product with a free trial and no sign-up fee.
3. Add it to the cart; observe the "recurring total" tax line shows $0.00 because the $0 initial calculation zeroed the shared tax rate.

Verifying the guard:

1. Run the suite: ./vendor/bin/phpunit --filter test_zero_amount_response_persists_real_itemized_rates — passes on trunk.
2. Temporarily change the line-item rate in get_itemized_tax_rates() to write the top-level rate — the test fails, confirming it catches the regression.

N/A - test-only change, no visual changes.

Checklist

• unit tests

• changelog.txt entry added — N/A: test-only change, no user-facing impact.
• readme.txt entry added — N/A: test-only change, no user-facing impact.

[bartech]

✅ LGTM — approving.

The regression test correctly locks in the zero-amount tax fix (WOOTAX-25): I traced the path and it genuinely fails if production regresses to writing the top-level shared tax_rate = 0, and CI is green across the PHP/WP/WC matrix.

A couple of optional, non-blocking hardening ideas:

• Consider also asserting $result['tax_rate'] is 0.0 — that's the field actually carrying the zeroed shared rate, and it's currently unasserted (the other half of the contract).
• A one-line comment on why assertCount( 4, … ) is load-bearing would help future readers.

Nice, well-scoped test.