add all simulate APIs

[adrian-y1]

Summary

Adds full coverage of Airwallex's sandbox Simulation API — the endpoints that stand in for a bank, card network, issuing bank, or shopper so async/manual lifecycles (deposits clearing, disputes escalating, KYC review outcomes, etc.) can be driven and tested without waiting on real-world events. Bumps the gem to 0.9.0.

What's new

New resources (sandbox Simulation only, no live create/retrieve/list yet):

• Deposit — .create/.retrieve/.list for real Direct Debit deposits, plus .simulate_create, .simulate_settle, .simulate_reject, .simulate_reverse
• IssuingTransaction — .simulate_create (with single_phase), .simulate_capture, .simulate_reverse, .simulate_refund, .simulate_notify_three_ds
• Cardholder — .simulate_pass_review
• LinkedAccount — .simulate_accept_mandate, .simulate_reject_mandate, .simulate_cancel_mandate, .simulate_fail_microdeposits
• AccountOffboarding — .simulate_complete, .simulate_cancel (nested under a parent ConnectedAccount)
• RFI — .simulate_create, .simulate_close, .simulate_follow_up (+ instance equivalents)
• POSTerminal — .simulate_turn_on, .simulate_turn_off, .simulate_generate_activation_code, .simulate_confirm_payment_intent, .simulate_payment_scenarios

Extended existing resources:

• Transfer — .simulate_transition/#simulate_transition
• Dispute — .simulate_create, .simulate_escalate/#simulate_escalate, .simulate_resolve/#simulate_resolve
• ConnectedAccount — .simulate_update_status/#simulate_update_status, #simulate_complete_offboarding, #simulate_cancel_offboarding
• AccountAmendment — .simulate_approve/#simulate_approve, .simulate_reject/#simulate_reject
• PaymentIntent — #simulate_shopper_pay, #simulate_shopper_reject
• PaymentConsent — #simulate_shopper_verify

Fixed

• RFI question answer.type corrected to the real uppercase enum ("TEXT", not "text") — confirmed against the live sandbox

Verified against the live sandbox

• Deposit.simulate_create, Transfer.simulate_transition, Dispute.simulate_create, RFI.simulate_create/#simulate_close, IssuingTransaction.simulate_create/.simulate_refund all confirmed working end-to-end
• IssuingTransaction.simulate_capture/.simulate_reverse and AccountAmendment.simulate_approve reach the correct endpoint (accepted, not 404) but couldn't be fully exercised on this sandbox account — both require Admin-level API key permissions this key doesn't have
• Identified (not fixed here) a pre-existing bug in Client's Faraday retry config: retry_options[:exceptions] doesn't include Faraday::RetriableResponse, so a POST that gets a retry_statuses code (429/500/502/503/504) raises uncaught instead of surfacing as a normal Airwallex::Error

Docs

• README's "Sandbox Simulations" section expanded to include a runnable example for every one of the 36 simulate_* methods added/touched in this PR, grouped into 10 subsections by resource
• CHANGELOG updated with the [0.9.0] entry

Test plan

• bundle exec rspec — 440 examples, 0 failures
• Live smoke test against sandbox (local_tests/smoke_test.rb) — read-only endpoints pass end-to-end
• Live simulation smoke test against sandbox (local_tests/simulation_smoke_test.rb) — exercises the new Simulation endpoints against real sandbox fixtures

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