adding message_id in the subscribe and on_subscribe APIs

[rajaneeshk90]

✨ Schema Enhancement: Addition of message_id in Registry Specification

Summary

This PR introduces a key schema update to the Beckn Registry Specification — the addition of the message_id field to improve message traceability and alignment with asynchronous communication patterns.

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✅ What’s Changed

1. Schema Enhancement – message_id Added

• message_id field (type: string) has been added to:
  • api/registry/build/registry.yaml
  • api/registry/components/io/OnSubscribe.yaml
  • schema/Subscription.yaml
• This enables better end-to-end correlation of messages exchanged between network participants.

2. Refactor of File Paths and Build Instructions

• Migrated registry compilation structure from core/v0/api/ to api/registry/.
• Updated paths and references in:
  • Schema index file
  • OpenAPI entry file (index.yaml)
  • Build command instructions using swagger-cli
• Ensured consistency in folder structure and YAML resolution flow.

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🔍 Why It Matters

• message_id is crucial for debugging and tracing workflows, especially in distributed or asynchronous messaging scenarios.
• The file structure refactor improves modularity and makes the spec more maintainable and intuitive.

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🧪 How It Was Tested

• Verified OpenAPI bundle using:

  swagger-cli bundle api/registry/components/index.yaml --outfile api/registry/build/registry.yaml --type yaml

[emmayank]

Subsequent changes are merged here - #488