Validate and centralize SERVICE_REGISTRY.yaml loading for runtime contract use

Summary

docs/SERVICE_REGISTRY.yaml is increasingly serving as a declarative contract for ACP/MCP flow behavior, but runtime access is still ad hoc and only lightly validated.

We recently added a small isolated helper for case_causal_review sanitization pass-through keys. That solved the immediate need without touching other services, but it exposed a broader improvement opportunity: service registry loading, validation, and typed access should be centralized.

Problem

Today the registry has these limitations:

• Runtime consumers can read YAML shape directly instead of going through a single validated access layer
• There is no schema validation on load
• Invalid or malformed sections may be silently ignored and replaced by fallbacks
• Flow-specific contract metadata such as sanitization pass-through keys can drift from tool signatures and docs
• Future additions will likely duplicate parsing logic unless we establish a shared pattern now

Goal

Create a validated, centralized service-registry access layer that can safely support runtime behavior, not just /services documentation.

Proposed scope

1. Introduce a shared service-registry helper/module as the single runtime access point
2. Validate SERVICE_REGISTRY.yaml structure on load
3. Provide typed helpers for common access patterns, for example:
   • service definition lookup
   • validation rules lookup
   • controlled identifier pass-through keys
   • declared MCP tool dependencies
4. Fail closed or emit explicit warnings for malformed registry sections
5. Add tests for:
   • valid registry load
   • malformed registry structure
   • missing file behavior
   • env override behavior via STUDY_AGENT_SERVICE_REGISTRY

Non-goals

• No full migration of every ACP/MCP caller in the first PR
• No broad schema redesign unless needed for validation
• No behavior changes to unrelated flows unless they opt into the helper

Motivation / examples

Concrete current use cases include:

• case_causal_review sanitization pass-through keys for controlled identifiers like ingred_rxcui and adverse_event_meddra_id
• service registration consistency between docs and runtime wiring
• future flow-specific validation metadata that should be declared once and consumed safely

Suggested implementation approach

• Start with a schema-light but strict validator
• Normalize known fields on load
• Keep conservative code-side fallbacks during rollout
• Migrate flows incrementally rather than all at once

Acceptance criteria

• Registry loading is centralized in one helper/module
• Malformed registry content is detected explicitly
• At least one existing runtime use case is powered by the validated helper
• Tests cover success, malformed input, env override, and fallback behavior