feat(agent-sdk): add getNativeBalance helper in TransactionUtil

[mehmetkr-31]

Summary

Adds getNativeBalance helper and GetNativeBalanceOptions to @xmtp/agent-sdk under sdks/js/agent-sdk/src/util/TransactionUtil.ts.

While TransactionUtil provides both ERC-20 (createERC20TransferCalls) and native token transfer helpers (createNativeTransferCalls), it previously only provided balance querying utilities for ERC-20 tokens (getERC20Balance, getERC20Decimals). This addition completes the API symmetry by allowing agents to query native token (ETH/MATIC) balances directly via viem's client.getBalance({ address }).

Changes

• TransactionUtil.ts: Added GetNativeBalanceOptions type and getNativeBalance(options: GetNativeBalanceOptions): Promise<bigint> implementation using viem createPublicClient.
• TransactionUtil.test.ts: Added unit test verifying native balance retrieval with mocked viem getBalance.

Verification

• yarn workspace @xmtp/agent-sdk run vitest run src/util/TransactionUtil.test.ts (11/11 tests pass)
• npx eslint agent-sdk/src/util/TransactionUtil.ts agent-sdk/src/util/TransactionUtil.test.ts (Clean)
• GPG signed commit.

Note

Add getNativeBalance helper to TransactionUtil in the agent SDK

Adds an exported getNativeBalance async function to TransactionUtil.ts that queries the native token balance (in wei) for a given address using viem's getBalance. It accepts a GetNativeBalanceOptions type with chain, address, and an optional transport (defaults to http()).

^{Macroscope summarized 00fe180.}