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/integration/test_config_override.py

@@ -52,7 +52,7 @@ def test_config_override_models_only(test_configs):
     # Run merge
     result = subprocess.run(
         [
-            "python", "-c",
+            "python3", "-c",
             f"import sys; sys.path.insert(0, '.'); "
             f"from tools.config_merger import DEFAULT_CONFIG_PATH, CUSTOM_CONFIG_PATH, OUTPUT_CONFIG_PATH, merge_and_validate; "
             f"import tools.config_merger; "
@@ -102,7 +102,7 @@ def test_config_validation_fails_gracefully(test_configs):
     # Run merge (should fail)
     result = subprocess.run(
         [
-            "python", "-c",
+            "python3", "-c",
             f"import sys; sys.path.insert(0, '.'); "
             f"from tools.config_merger import merge_and_validate; "
             f"import tools.config_merger; "