Add unit tests for TransactionResult round-trip serialization

[Emrys02]

Verifying that TransactionResult decodes and re-encodes identically maintains parsing reliability for RPC response integrations.

Implementation Guidelines

Key Files: crates/core/src/xdr/codec.rs (in the tests module).

• Add functional tests ensuring a TransactionResult safely completes a full codec round-trip.

Expectations

What done looks like: Complete test coverage for TransactionResult serialization functions.

For any question, reach me on Discord: emry_ss

[samjay8]

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