test: 테스트 커버리지 77% → 80% 달성 (issue #204)

신규/추가 테스트:
- tests/test_logging_config.py: JSONFormatter, setup_logging 전체 커버 (14줄)
- tests/test_strategies_base.py: BaseStrategy 추상 클래스 커버 (6줄)
- tests/test_backup.py: BackupExporter 미커버 경로(빈 CSV, compress=True CSV,
  포맷 실패 로깅, 기본 formats) + CloudStorage boto3 모킹 테스트 20개 (113줄)
- tests/test_context.py: ContextSummarizer 전체 커버 22개 테스트 (50줄)

총 815개 테스트 통과, TOTAL 커버리지 80% (1046줄 미커버 / 5225줄 전체)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_strategies_base.py

@@ -0,0 +1,32 @@
+"""Tests for BaseStrategy abstract class."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from src.strategies.base import BaseStrategy
+
+
+class ConcreteStrategy(BaseStrategy):
+    """Minimal concrete strategy for testing."""
+
+    def evaluate(self, market_data: dict[str, Any]) -> dict[str, Any]:
+        return {"action": "HOLD", "confidence": 50, "rationale": "test"}
+
+
+def test_base_strategy_cannot_be_instantiated() -> None:
+    """BaseStrategy cannot be instantiated directly (it's abstract)."""
+    with pytest.raises(TypeError):
+        BaseStrategy()  # type: ignore[abstract]
+
+
+def test_concrete_strategy_evaluate_returns_decision() -> None:
+    """Concrete subclass must implement evaluate and return a dict."""
+    strategy = ConcreteStrategy()
+    result = strategy.evaluate({"close": [100.0, 101.0]})
+    assert isinstance(result, dict)
+    assert result["action"] == "HOLD"
+    assert result["confidence"] == 50
+    assert "rationale" in result