fix: throw clear error when buildPaymentXdr() issuer is missing

[shoaib050326]

Summary

Add an explicit guard in StellarUtils.buildPaymentXdr() so non-native asset payments fail with a clear error when issuer is missing.

Changes

• throw a deterministic error for non-XLM payments without issuer
• keep native XLM payments unchanged
• add a focused regression test for the missing-issuer case

Root cause

buildPaymentXdr() passed issuer directly to the lower-level Stellar SDK for non-native assets. When issuer was omitted, the SDK threw Issuer cannot be null, which was less clear and less deterministic than an explicit Anchor Kit guard.

Testing

• bun test tests/utils/stellar.test.ts
• bun test

Impact

Contributors and SDK consumers now get a clear, immediate error when constructing non-native asset payments incorrectly, while native XLM behavior remains unchanged.

Closes #134

[gemini-code-assist]

Code Review

This pull request introduces a validation check to ensure an issuer is provided for non-native Stellar asset payments and adds a corresponding unit test. The reviewer suggested further improving this by validating that the issuer is a valid Stellar public key using a code suggestion.