chore/testnet-deploy-metadata-registry-contract

[Wetshakat]

closed #868

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Title: feat: Implement testnet metadata registry contract

Description:

This PR introduces the metadata_registry smart contract to provide an on-chain repository for storing active contract metadata. This registry serves as a verifiable source of truth for our testnet deployment set, aligning with our current deployment documentation flow.

Key Changes:

• Metadata Registry Contract (contracts/metadata_registry): Added a new Soroban contract that stores metadata mappings for ecosystem contracts (address, version, environment, and last updated timestamp).
• Authenticated Admin Updates: Added admin.require_auth() checks to the set_metadata function to ensure only the authorized administrator can make registry updates.
• Observability: The contract emits InitializedEvent and MetadataUpdatedEvent to allow off-chain indexers and clients to track ecosystem footprint changes in real-time.
• Client Query Interfaces: Implemented get_metadata(key) and get_all_contracts() functions for clients to easily query and verify the active testnet deployment map.
• Workspace Integration: Registered metadata_registry into the apps/onchain/Cargo.toml workspace members list.
• Test Coverage: Added unit tests verifying standard registry flow and unauthorized rejection cases (all passing).

Note for Reviewers:
When the next testnet deployment goes out, we will also need to generate a manifest ID for this registry and bump the deployed contract count audit within our testnet-manifest.json and validate-manifest.js script.

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[Cedarich]

@Wetshakat fix workflow