sdk: add batchMint wrapper

[artylobos]

Summary

• add the SDK batchMint(recipients, source) wrapper using the requested { to, amount } recipient objects
• export the BatchMintRecipient type and document batch mint usage in the SDK README
• add coverage for the real client argument encoding and the in-memory mock client
• fix SDK compile blockers that prevented the updated client and utility files from building

Validation

• cd sdk && npm run build
• cd sdk && npm test -- --runInBand
• cd sdk && npx eslint src/client.ts src/client.test.ts src/events.ts src/index.ts src/mockClient.ts src/mockClient.test.ts src/utils.ts (0 errors; existing explicit-any warnings remain)

Closes #74

Bounty Note: Stellar Wave 5 — 200 Points

[artylobos]

Local validation completed because the forked PR workflow is currently action_required and has no jobs yet:

• cd sdk && npm run build
• cd sdk && npm test -- --runInBand
• cd sdk && npx eslint src/client.ts src/client.test.ts src/events.ts src/index.ts src/mockClient.ts src/mockClient.test.ts src/utils.ts returned 0 errors; existing explicit-any warnings remain.

The PR is ready for maintainer review once Actions are approved.