test: improve test coverage from 61% to 84.81%

Major improvements:
- Fixed all 42 broken tests (database connection leaks)
- Added db_connection() context manager for proper cleanup
- Created comprehensive test suites for undertested modules

New test coverage:
- tools/general_tools.py: 26 tests (97% coverage)
- tools/price_tools.py: 11 tests (validates NASDAQ symbols, date handling)
- api/price_data_manager.py: 12 tests (85% coverage)
- api/routes/results_v2.py: 3 tests (98% coverage)
- agent/reasoning_summarizer.py: 2 tests (87% coverage)
- api/routes/period_metrics.py: 2 edge case tests (100% coverage)
- agent/mock_provider: 1 test (100% coverage)

Database fixes:
- Added db_connection() context manager to prevent leaks
- Updated 16+ test files to use context managers
- Fixed drop_all_tables() to match new schema
- Added CHECK constraint for action_type
- Added ON DELETE CASCADE to trading_days foreign key

Test improvements:
- Updated SQL INSERT statements with all required fields
- Fixed date parameter handling in API integration tests
- Added edge case tests for validation functions
- Fixed import errors across test suite

Results:
- Total coverage: 84.81% (was 61%)
- Tests passing: 406 (was 364 with 42 failures)
- Total lines covered: 6364 of 7504

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/api/test_period_metrics.py

@@ -52,3 +52,32 @@ def test_calculate_period_metrics_negative_return():
     assert metrics["calendar_days"] == 8
     # Negative annualized return
     assert metrics["annualized_return_pct"] < 0
+
+
+def test_calculate_period_metrics_zero_starting_value():
+    """Test period metrics when starting value is zero (edge case)."""
+    metrics = calculate_period_metrics(
+        starting_value=0.0,
+        ending_value=1000.0,
+        start_date="2025-01-16",
+        end_date="2025-01-20",
+        trading_days=3
+    )
+
+    # Should handle division by zero gracefully
+    assert metrics["period_return_pct"] == 0.0
+    assert metrics["annualized_return_pct"] == 0.0
+
+
+def test_calculate_period_metrics_negative_ending_value():
+    """Test period metrics when ending value is negative (edge case)."""
+    metrics = calculate_period_metrics(
+        starting_value=10000.0,
+        ending_value=-100.0,
+        start_date="2025-01-16",
+        end_date="2025-01-20",
+        trading_days=3
+    )
+
+    # Should handle negative ending value gracefully
+    assert metrics["annualized_return_pct"] == 0.0