test: add totals integrity conformance module

[vishkaty]

What

Adds totals_test.py, a conformance module for the structural invariants of the
totals[] array. business_logic_test.py checks totals arithmetic (via
existence lookups), but nothing asserts the cardinality and sign rules
the spec places on the array itself:

Test	Requirement (types/totals.json, types/total.json)
test_single_subtotal_entry	exactly one subtotal entry (contains + minContains:1/maxContains:1)
test_single_total_entry	exactly one total entry
test_entries_have_type_and_amount	every entry carries type and amount (required)
test_additive_entries_non_negative	additive well-known types (subtotal, fulfillment, tax, fee) have amount >= 0
test_discount_entry_is_negative	subtractive well-known types (discount, items_discount) have amount < 0

Why

Cardinality ("exactly one subtotal/total") and per-type sign are load-bearing
for any platform rendering a receipt, and the SDK model only enforces presence
of type/amount, not these array-level invariants — so a server can regress
them and still pass. These cut across checkout, fulfillment, and discount
responses.

Design

Server-agnostic relative assertions read from the raw response (consistent with
the other modules). test_entries_have_type_and_amount asserts on the raw wire
(int amount, non-empty type) rather than the parsed model, so it catches what
the model's lax coercion would mask. Sub-line (lines[]) and per-entry-currency
cases are intentionally omitted — the spec's totals amount is a bare
signed_amount (no currency), and the reference emits no sub-lines, so testing
them would be vacuous.

Verification

• New module green against the reference; full suite green alongside it (14
  files, no regression).
• pre-commit run clean (ruff, ruff-format, codespell).