ci: add Vitest unit tests for stellar.ts and operationDefinitions

[cokehill]

Add unit test coverage for the core library logic.
Acceptance criteria:
[ ] Tests for isValidPublicKey, isValidSecretKey
[ ] Tests for parseAsset (native, alphanum4, alphanum12, invalid)
[ ] Tests for buildOperation for at least Payment, CreateAccount, ChangeTrust
[ ] Tests for all entries in OPERATION_DEFINITIONS to confirm no missing fields
[ ] CI passes on every push (already configured in .github/workflows/ci.yml)