feat: implement 'requires_escalation' triggered by risk signals

[0x8000-0000]

Description

Why:
- To enable conformance testing of the checkout escalation flow according to spec.
- Provides a deterministic way to trigger 'requires_escalation' via 'risk_signals' payload in 'complete_checkout'.

What:
- Removed hardcoded buyer name check.
- Updated 'complete_checkout' to check for 'simulation_trigger': 'escalation_required' in 'risk_signals'.
- When triggered, returns 200 OK with status 'requires_escalation', 'continue_url', and 'requires_buyer_input' message, instead of completing the order.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules - this depends on feat: implement 'requires_escalation' triggered by risk signals samples#25 which is not merged yet.