fix: validate payment XDR account addresses

[shoaib050326]

What does this PR do?

Adds upfront validation for payment XDR account addresses in StellarUtils.buildPaymentXdr(), so invalid source and destination inputs fail early with clear errors while still accepting valid muxed (M...) addresses supported by the Stellar SDK.

How to test?

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

You can also verify manually that:

• invalid source rejects with source must be a valid Stellar public or muxed public key
• invalid destination rejects with destination must be a valid Stellar public or muxed public key
• valid muxed source and destination addresses still build successfully

Checklist

• My code follows the code style of this project.
• I have added tests for my changes.
• I have run bun run test and bun run lint locally.

Issue Reference

Closes #135

[gemini-code-assist]

Code Review

This pull request introduces upfront validation for Stellar source and destination addresses in the buildPaymentXdr function, adding support for muxed accounts and corresponding unit tests. Review feedback suggests extending this validation to the issuer field for non-native assets and simplifying the sourceAccount instantiation by using MuxedAccount.fromAddress directly, as it handles both standard and muxed addresses.