Zero Trust and Architectural Integrity Refactor

[dcplatforms]

This submission strengthens the OCP SDK's adherence to the "Five Pillars," specifically Architectural Integrity and Security via Zero Trust.

Key changes include:

1. A2AService Refactor: Aligned with the repository pattern using the centralized database adapter and implemented cryptographic mandate validation for Agent-to-Agent transfers.
2. Zero Trust Error Standardization: Enforced the mandatory "Zero Trust Validation Failed: " prefix across all fiduciary and policy validation failures in AgentService, MandateService, and UCPService.
3. Observability Enhancements: Standardized TokenizationService to use the _handleError(method, error) pattern, improving log context.
4. UCP Mandate-Awareness: Modified UCPService.processPayload to propagate mandates to the underlying transfer logic, completing the "Chain of Evidence" for the Agentic Economy.
5. Testing: Comprehensive verification with 10/10 passing unit tests and successful execution of integration verification scripts.

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PR created automatically by Jules for task 6176282475414098330 started by @dcplatforms

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