test: add discount capability conformance module

[vishkaty]

What

Adds discount_test.py, a conformance module for the discount capability
(dev.ucp.shopping.discount). It complements the discount coverage already in
business_logic_test.py (discount flow, multiple-code accept/reject,
fixed-amount) by asserting discount MUSTs from discount.md that the suite does
not yet check:

Test	Requirement
test_code_matches_case_insensitively	codes are matched case-insensitively
test_discount_total_is_negative	a discount's totals[] entry is negative (total.json exclusiveMaximum: 0)
test_codes_replace_previous_set	a new codes[] set replaces the previous one
test_empty_codes_removes_discount	an empty codes[] removes all discounts
test_allocations_sum_to_applied_amount	an applied discount's allocations[] sum to its amount (a cross-field invariant the JSON schema cannot express)
test_client_applied_does_not_change_price	client-supplied discounts.applied[] (response-only, ucp_request: omit) does not change the priced total

Why

These are the discount behaviours most likely to differ silently between
implementations — case-folding, the sign convention on totals[], code-set
replacement vs. accumulation, and the allocation/amount reconciliation. They are
currently unverified at the conformance level, so a merchant can regress them
and still pass. The last test also locks in that discounts are priced
server-side: a client cannot inject a cheaper order via the response-only
applied[] field.

Design

Consistent with the conformance_input-driven approach in #58: assertions are
relative (a discount reduced the total; the applied code is echoed; the
discount total is negative; an empty set clears it; allocations reconcile)
rather than tied to a specific discount value, so any conformant business passes
regardless of the discount amounts it offers. Codes are read from
conformance_input.json under test_fixtures
(valid_discount_code / second_discount_code). The module skips honestly when
no code is configured, the business does not advertise the capability, or the
server emits no allocations[].

Verification

• All 6 tests green against the reference Flower Shop server.
• Full suite green alongside the new module (no regression).
• pre-commit run --all-files clean (ruff, ruff-format, prettier, codespell).