build out further endpoints for gem

[iivancic-1]

Summary

Adds the remaining Airwallex API resources needed for Winboard's integration (Global Accounts, Billing/Subscriptions, Payment Consents/Sources, Connected Accounts, Funds Splits, Charges, Account Amendments) and fixes several pre-existing bugs surfaced by testing against a real sandbox account — most notably that the shared Update and Delete mixins were sending the wrong HTTP verb to the wrong URL for every resource that used them.

Added

• GlobalAccount — create, retrieve, list, update, #close, #generate_statement_letter, #transactions
  • GlobalAccountAlias (nested) — #create_alias, #alias, #aliases, #initiate_port, #submit_verification_code, #request_new_verification_code, #cancel
  • GlobalAccountMandate (nested) — #mandate, #mandates, #cancel
• Billing resources — BillingCustomer, BillingProduct, BillingPrice, BillingSubscription (with #items/#item)
• PaymentConsent — create, retrieve, list, update, #verify, #verify_continue, #disable
• PaymentSource — create, retrieve, list
• ConnectedAccount — create, retrieve, list, update, #submit, #agree_to_terms_and_conditions, #suspend, #reactivate, .current, .wallet_info, #legal_entity_id
• AccountAmendment — create, retrieve
• FundsSplit — create, retrieve, list, #release
• Charge — create, retrieve, list
• Beneficiary gained .update/#update, .validate, .verify_account, .api_schema, .form_schema, and .supported_financial_institutions
• Dispute gained .update, #challenge (replacing #submit_evidence), and #related_payment_intents
• PaymentMethod gained #disable
• New tests covering all of the above (378 total)

Fixed

• APIOperations::Update now sends POST #{resource_path}/{id}/update instead of PUT #{resource_path}/{id}
• APIOperations::Delete now sends POST #{resource_path}/{id}/delete instead of DELETE #{resource_path}/{id}, and checks the response instead of always returning true
• Dispute's resource path corrected from /api/v1/disputes to /api/v1/pa/payment_disputes
• Conversion's resource path corrected from /api/v1/conversions to /api/v1/fx/conversions
• Configuration#api_version now defaults to nil instead of a hardcoded date, and x-api-version is only sent when explicitly set
• Beneficiary.create/.validate/.update payloads corrected: wrapped under a top-level beneficiary key, with nickname/payer_entity_type/transfer_methods/transfer_reason as top-level siblings; entity_type replaces the nonexistent beneficiary_type
• Beneficiary#update/.update requires the full payload, not a partial patch
• Beneficiary.supported_financial_institutions requires account_currency, entity_type, transfer_method, and keyword in addition to bank_country_code
• Customer.create requires merchant_customer_id
• GlobalAccount#generate_statement_letter requires account_statement_type and registration_info
• ConnectedAccount.create payload corrected: account_details is a required top-level wrapper; legal_entity_type is "BUSINESS"/"INDIVIDUAL", not "COMPANY"
• BillingCustomer.create payload corrected to match the real flat schema (name/email/type/default_billing_currency/address/default_legal_entity_id)
• BillingPrice.create requires pricing_model and recurring
• BillingSubscription.create payload corrected: prices attach via an items: array, not a flat price_id; starts_at replaces start_date; added legal_entity_id and payment_source_id
• AccountAmendment.create payload corrected to use target plus the changed section as a top-level sibling, not a generic changes: wrapper
• PaymentSource.create's external_id corrected to reference a PaymentMethod id, not a PaymentConsent id
• ConnectedAccount#legal_entity_id added as the source for BillingCustomer/BillingSubscription's legal_entity_id, rather than a separate resource
• Rate/Quote use sell_currency/buy_currency, not from_currency/to_currency
• Quote.create requires validity (MIN_1, MIN_15, MIN_30, HR_1, HR_4, HR_8, or HR_24)

Removed

• Rate.list — the endpoint has no "list all rates" concept, only .retrieve(sell_currency:, buy_currency:)
• PaymentMethod.delete and PaymentMethod#detach — replaced by #disable, the real lifecycle operation

Test plan

• bundle exec rspec — 378 examples, 0 failures
• bundle exec rubocop — no offenses
• Manually verified against a real Airwallex sandbox account for the resources above

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