feat: add X-Request-ID header support for idempotency (#3)

[dapi]

Summary

• Add request_id: keyword parameter to payment methods for idempotency support
• The X-Request-ID header is sent with API requests when request_id is provided
• Enables CloudPayments server-side result caching for 1 hour
• Fully backward compatible — request_id: is optional

API

# Charge with idempotency
client.payments.tokens.charge(attributes, request_id: "charge:#{invoice_id}")

# Refund with idempotency
client.payments.refund(transaction_id, amount, request_id: "refund:#{transaction_id}")

# Subscription with idempotency
client.subscriptions.create(attributes, request_id: "subscription:#{account_id}")

Changes

File	Change
lib/cloud_payments/client.rb	Add request_id: to perform_request and headers
lib/cloud_payments/namespaces/base.rb	Add request_id: to request method
lib/cloud_payments/namespaces/*.rb	Add request_id: to charge/auth/refund/create methods
spec/cloud_payments/client_spec.rb	Test X-Request-ID header behavior
README.md	Documentation with usage examples
Gemfile	Add base64 gem for Ruby 3.4+ compatibility

Design Decision

Used explicit request_id: parameter instead of Thread.current block approach:

• Avoids Fiber-local storage issues in async environments (Falcon, async-http)
• More explicit and predictable API
• Ruby 3.0+ compatible

Test plan

• All existing tests pass (486 examples, 0 failures)
• New tests for X-Request-ID header presence/absence
• Tests verify empty string and nil are not sent as header

Closes #3

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