Test: add RAM-billing and KV API coverage tests

[heifner]

Adds 38 BOOST_FIXTURE_TEST_CASE entries in kv_api_tests and 2
BOOST_AUTO_TEST_CASE entries (kv_roa_limit_enforcement,
privileged_kv_payer_bypass) in protocol_feature_tests covering
payer-change billing, erase refund attribution, secondary-index
billing, cross-account billing, read-only rejection, payer
authorization edge cases, notification billing, mixed-payer
transactions, ROA quota enforcement, privileged-sysio bypass,
key/value size boundaries, iterator invalidation under mutation,
and cross-contract secondary reads.

Test coverage

Category	Tests	What it validates
Payer change billing	4	Correct refund/charge on payer change (same size, grow, shrink, back-to-self)
Erase refund	1	Refunds stored payer, not receiver
Secondary index billing	4	Correct amounts for idx store/remove/update
Cross-account idx billing	3	Idx bills payer (not contract), refunds correctly
Read-only rejection	4	erase, idx_store/remove/update blocked (table_operation_not_permitted)
Payer auth edge cases	3	Old payer unauth ok, missing auth fails, active-only (no sysio.payer) fails
Notification billing	2	Self-pay ok, third-party blocked in notify
Mixed-payer	2	Independent deltas, net zero
ROA limit	1	Quota exceeded -> ram_usage_exceeded
Privileged bypass	1	sysio.* bypasses payer auth, non-sysio.* does not
Key/value boundaries	6	Max and oversize on both create and update
Iterator invalidation	6	Primary + secondary mutation during iteration
Cross-contract reads	1	Secondary index read via kv_idx_primary_key + kv_get across contracts