feat(sdk)!: flatten request body arguments

[matoous]

Update the Python SDK generator to expose declared top-level JSON request body fields as keyword-only
method arguments instead of requiring explicit body= model construction, and regenerate the SDK with the
new signatures. Refresh the README and examples to use the flattened call style so the public usage
matches the generated API.

Migration Guide

Request body objects are now flattened into method keyword arguments. You no longer need to construct/
pass the generated request body type explicitly.

Before:

sdk.subaccounts.update_sub_account(
  1,
  body=UpdateSubAccountBodyInput(
      nickname="New name",
  ),
)

After:

sdk.subaccounts.update_sub_account(
  1,
  nickname="New name",
)

Required body fields are now required keyword arguments:

sdk.subaccounts.create_sub_account(
  username="operator@example.com",
  password="correct horse battery staple",
)

Optional flattened request-body parameters now distinguish omitted fields from explicit JSON null.

If you omit an optional body argument, the SDK omits it from the JSON payload:

sdk.subaccounts.update_sub_account(1)
# Sends: {}

If you pass None, the SDK sends an explicit JSON null:

sdk.subaccounts.update_sub_account(1, nickname=None)
# Sends: {"nickname": null}

For query parameters, omitted parameters and None are both omitted from the query string:

sdk.transactions.list("merchant-123")
# No optional query params sent

sdk.transactions.list("merchant-123", limit=None)
# "limit" is omitted