fix: use configured fixed-discount amount in business_logic_test

[XiaolongZhang-TT]

Description

test_fixed_amount_discount in business_logic_test.py fetched the expected reduction into expected_discount and already passed it to assert_totals_consistent(...), but the separate assertion on discounts.applied[0].amount hardcoded 500 instead of using that variable:

expected_discount = self.fixture_ctx.get_expected_fixed_discount_reduction()
...
self.assert_totals_consistent(..., expected_discount=expected_discount)   # uses it
...
self.assertEqual(discounts_obj.applied[0].amount, 500)                    # ignores it

get_expected_fixed_discount_reduction() already returns the value in minor units (expected_fixed_discount_reduction major units × 100), so the hardcoded 500 only matched the default flower_shop fixture (5.00 → 500) by coincidence. Any non-default fixture (e.g. a configured 7.50 fixed reduction → expected 750) would pass the totals-consistency check but fail the applied[0].amount assertion for the same checkout, an internal contradiction.

Fix: assert against expected_discount, making the two assertions in the same test consistent and server-agnostic.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, including removal of schema files or fields)
• Documentation update

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Is this a Breaking Change or Removal?

N/A — test-only fix, no schema/field removal.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A — test-only fix)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[damaz91]

Nice catch, thanks @XiaolongZhang-TT !