[Feature/Roadmap] Implement ACP checkout support (Agentic Commerce Protocol)

[SergeevDmitry]

Context & Rationale

The gateway currently supports single-request commerce: invoke one resource,
settle one price and receive one result. ACP
adds a checkout model for agents that assemble and complete a purchase across
multiple requests.

Supporting ACP makes the gateway useful for merchants selling products through
agent-driven checkout, while the existing execution pipeline remains the only
path to final delivery.

Proposed Changes

• Add an ACP adapter for the core checkout lifecycle: create, read, update,
  complete and cancel a checkout.
• Persist checkout state behind an ACP-owned store and define expiry and
  terminal-state behavior.
• Map checkout completion to exactly one execution, delivery and commerce
  receipt, with idempotent handling of retries.
• Forward delegated payment data to the merchant's existing payment flow
  without storing or charging it in the gateway.
• Declare the ACP capabilities the deployment actually supports and report the
  same scope through configuration, diagnostics and documentation.
• Add protocol-level tests for successful checkout, cancellation, retries,
  invalid state transitions and unsupported capabilities.

Non-goals / Invariants Preserved

• Product feeds, catalog synchronization, discounts, refunds, returns and
  post-delivery order changes are deferred.
• The merchant remains the merchant of record; the gateway does not custody
  funds or payment credentials.
• ACP-specific state and types do not become part of the canonical commerce
  model.
• Checkout completion must not bypass the shared execution pipeline or produce
  duplicate delivery on retry.

Impact & Value

• Extends the project from paid API-style calls to agent-driven retail checkout.
• Gives merchants a standards-based integration while preserving their current
  backend and payment-provider relationships.
• Establishes the state and idempotency pattern needed for future session-based
  commerce protocols.