feat(merkle_bridge): integrate carbon_asset minting cross-contract call (#521) - #585

Summary

This PR completes the deferred cross-contract integration between merkle_bridge and carbon_asset. Previously, mint_wrapped performed Merkle proof verification but only incremented an isolated local counter instead of minting actual tokens on carbon_asset.

With this change, mint_wrapped consumes the configured CarbonAssetContract address, executes a guarded cross-contract mint call, and persists/emits the real canonical token ID returned by carbon_asset.

Changes Made

• WASM Import: Imported carbon_asset.wasm into merkle_bridge/src/lib.rs via contractimport!.
• Error Handling: Added MerkleBridgeError::MintFailed (code 10) to handle trapped or non-success cross-contract calls safely.
• Cross-Contract Minting:
  • Refactored mint_wrapped to fetch the configured address using get_carbon_asset_contract.
  • Replaced local NextTokenId generation with env.try_invoke_contract calling carbon_asset::mint.
  • Mapped call failures to MerkleBridgeError::MintFailed.
• State & Events: Updated CreditBridgedEvent and local storage mappings to record the canonical token_id returned by carbon_asset. Added get_token_id helper.
• Test Suite & Tooling:
  • Added unit and integration tests covering successful minting, missing CarbonAssetNotSet configuration, and cross-contract MintFailed errors.
  • Retained full regression coverage for existing Merkle proof verification logic.
  • Added build-carbon-asset-wasm.ps1 helper script for fixture builds.

Verification

• Run cargo test in stellar-core/verifiable-registry/contracts/merkle_bridge/ to verify all proof verification, error mapping, and contract integration tests pass.
• Confirmed zero local-only token ID counters remain disconnected from carbon_asset.

Closes #521