test(stellar): add comprehensive unit test coverage for transaction builders (#45)

[ghzhost]

Summary

Closes #45

Adds full unit test coverage for , covering all key transaction construction, submission, and formatting logic.

Test Coverage Details

1. Transaction Builders:
   • : verifies native & issued payment operation building, destination key validation, amount precision, fee scaling, timeout configuration, and memo type handling (detects text vs numeric ID).
   • : verifies construction with destination minimums under slippage, path hops mapping, and parameters.
   • : routes correctly to standard payment vs path payment based on quote path length, applying configured slippage tolerance.
   • : ensures linear fee scaling per recipient, no exponential fee regression, and strict validation of recipient counts (0 and > MAX_BATCH_RECIPIENTS).
2. Horizon Submission:
   • : validates signed transaction XDR forwarding to Horizon server.
3. Asset & Validation Utilities:
   • , , : tests native handling and missing issuer errors on non-native assets.
   • , , , , , .
4. Environment & Server Selectors:
   • , , and (including network failure fallback to ).

Verification

• : 27/27 test files passed, 128/128 tests passed.
• (Version 5.4.5
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