[fix][relayer][core] preserve blockchain properties on partial update

[fengjy73]

CLI/API updates may provide only a subset of blockchain properties, such as heterogeneous_bbc_context. The previous flow merged the partial config into the existing BlockchainMeta, but then persisted a newly constructed meta from only the partial properties. That could wipe fields that were not present in the update payload.

Affected fields include plugin server metadata, init block height, AM/SDP/PTC contract addresses, and domain registration state.

• Persist the merged BlockchainMeta in BlockchainManager.updateBlockchain(...).
• Merge ptc_contract_address in BlockchainMeta.updateProperties(...).

[zouxyan]

Reviewed commit 132ad53. Persisting the merged BlockchainMeta correctly preserves fields omitted from partial updates, and the added PTC contract address merge is consistent with the existing AM/SDP behavior. No blocking correctness issues found.