test: add test helpers, docs, and regression tests

[rayvorden]

What

Comprehensive test improvements including a balance computation helper, contract versioning documentation, sell event regression test, and initialized flag unit tests.

Issues Resolved

Closes #342
Closes #343
Closes #344
Closes #345

Changes

#342 - Add helper for computing expected key balance after mixed buy and sell sequence

• Add TradeOperation enum (Buy/Sell) for representing trade sequences
• Implement compute_expected_balance_after_trades helper in contract_test_env
• Create comprehensive test suite (balance_after_mixed_trades.rs) with three test cases
• Tests verify balance tracking with various buy/sell sequence patterns
• Helper makes test intent explicit and reduces maintenance burden on complex fixtures

#343 - Add docs for contract versioning approach and WASM hash tracking

• Document semantic versioning approach and release numbering strategy
• Explain WASM hash as deployment identifier and version indicator
• Provide instructions for retrieving contract WASM hash from Stellar blockchain
• Include build verification procedures and determinism considerations
• Document canonical hash recording and verification workflow
• Add best practices for operators, contributors, and users
• Include emergency procedures for hash mismatch scenarios

#344 - Add regression test for sell event containing correct seller address

• Create dedicated test file (sell_event_seller_address.rs) focusing on seller address integrity
• Test verifies seller address field in sell event matches call initiator
• Assert event field is present and contains correct address value
• Separate tests for address match and field presence validation

#345 - Add unit tests for protocol config initialized flag read after setup

• Add test verifying initialized flag is false before any initialization
• Covers pre-initialization state as distinct test case
• Complements existing post-initialization test
• Uses consistent setup conventions throughout

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