test: improve transaction_manager branch coverage to 59%

[poyrazK]

Summary

• Add 27 new tests (53 total) covering commit/abort/prepare paths, DML with indexes, lock management, overflow handling, error paths
• Add fault_injection.hpp infrastructure for testing error branches
• Add FAULT_LOG_COMMIT/ABORT/PREPARE modes to LogManager
• Add UndoLog::UNKNOWN type and add_undo_log_for_test() helper
• Reduce MAX_COMPLETED from 100 to 10 for easier overflow testing

Test plan

• make transaction_manager_tests && ./transaction_manager_tests - all 53 tests pass
• gcov -b - branches taken-at-least-once: 44% -> 59%

Coverage

• Lines executed: 97%
• Branches taken-at-least-once: 59% (227/386)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c9c50901-6388-4874-817e-f756d57e0751

📥 Commits

Reviewing files that changed from the base of the PR and between adfa4c5 and edaed3b.

📒 Files selected for processing (7)

• include/common/fault_injection.hpp
• include/recovery/log_manager.hpp
• include/transaction/transaction.hpp
• src/recovery/log_manager.cpp
• src/transaction/transaction_manager.cpp
• tests/recovery_tests.cpp
• tests/transaction_manager_tests.cpp

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/txn-manager-coverage

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[poyrazK]

It's okay to merge