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>

api/migrations/001_trading_days_schema.py

@@ -66,7 +66,7 @@ def create_trading_days_schema(db: "Database") -> None:
             completed_at TIMESTAMP,
 
             UNIQUE(job_id, model, date),
-            FOREIGN KEY (job_id) REFERENCES jobs(job_id)
+            FOREIGN KEY (job_id) REFERENCES jobs(job_id) ON DELETE CASCADE
         )
     """)
 
@@ -101,7 +101,7 @@ def create_trading_days_schema(db: "Database") -> None:
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             trading_day_id INTEGER NOT NULL,
 
-            action_type TEXT NOT NULL,
+            action_type TEXT NOT NULL CHECK(action_type IN ('buy', 'sell', 'hold')),
             symbol TEXT,
             quantity INTEGER,
             price REAL,