fix(BTI-1191): preserve internal casing of top-level service parameter names

[vildanbina]

Problem

add_parameter flattened every parameter name with str.capitalize(), which uppercases the first letter and lowercases the rest. A camelCase key like customerFirstName went on the wire as Customerfirstname. Przelewy24 and Trustly are case-sensitive and reject that, so their Pay requests failed with missing customer parameters (BTI-1191).

Present since the builders were first written (2026-01-21); only surfaced now because BTI-1191 added the first case-sensitive camelCase params to those methods.

Fix

Top-level parameter names now keep their internal capitals via _upper_first (first letter up, rest untouched). Grouped names (group_type set: In3/Billink/Riverty/Klarna and credit-management customer blocks) keep the legacy .capitalize() flatten, which is the gateway-verified form for those.

One chokepoint in base_builder.add_parameter, so it fixes every method's top-level params at once (Przelewy24, Trustly, plus latently-miscased Klarna, Credit Management, eMandate, Marketplaces, Banking, Giftcards, SessionId, PaymentData, payPalOrderId).

Tests

• Full SDK suite green (2503 passed; the 4 logging_observer failures are pre-existing and environmental, unrelated to this change).
• Updated the assertions that pinned the old flattened names to the correct PascalCase.

E2E (Odoo dev store, live gateway)

• Przelewy24: reaches gateway + transaction key, no missing-params error (was the failure).
• Credit Card (SessionId): authorize -> capture -> done.
• Billink, Riverty (grouped path): full success, unchanged.
• iDEAL: full success (regression baseline).
• Trustly: request reaches gateway with customer data; test gateway returns a separate S996 decline (not this bug).

Risk

Changes request param casing for several live methods. Each currently works in production with the flattened name, so their gateways are case-insensitive (change is a no-op) or were latently miscased (change fixes it). A sandbox check on Klarna and the solutions builders before release is recommended.