Audit and verify Engine testing coverage

All tests pass and lint is clean. Let me verify the final test coverage

**Engine.hs Test Coverage (13 tests):** - ✅ Tool JSON roundtrip -
✅ Message JSON roundtrip - ✅ ToolCall JSON roundtrip (NEW) -
✅ FunctionCall JSON roundtrip (NEW) - ✅ Role JSON roundtrip
for all roles (NEW) - ✅ defaultLLM endpoint & headers - ✅
defaultAgentConfig defaults - ✅ defaultEngineConfig callbacks - ✅
buildToolMap correctness - ✅ Usage JSON parsing - ✅ AgentResult
JSON roundtrip - ✅ estimateCost calculation

**Tools.hs Test Coverage (19 tests):** - ✅ All 5 tool schemas are
valid objects - ✅ allTools contains 5 tools - ✅ ReadFileArgs
parsing - ✅ WriteFileArgs parsing - ✅ EditFileArgs parsing - ✅
RunBashArgs parsing - ✅ SearchCodebaseArgs parsing - ✅ ToolResult
success/failure JSON roundtrip - ✅ readFileTool handles missing files
(NEW) - ✅ editFileTool handles no-match case (NEW) - ✅ runBashTool
captures exit codes (NEW) - ✅ runBashTool captures stdout (NEW) -
✅ searchCodebaseTool returns structured results (NEW)

All unit tests from the checklist are now covered. The integration
and m

Task-Id: t-141.7

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