Raise coverage threshold to 95% and expand test coverage

- pyproject.toml: fail_under 80 → 95
- test_alpha: +79 lines
- test_cli_workflow: +226 lines
- test_derived: +121 lines
- test_downloader_contracts: +169 lines
- test_features: +16 lines
- test_minute_downloader: +81 lines
- test_portfolio: +208 lines

pyproject.toml

@@ -39,7 +39,7 @@ source = [
 ]
 
 [tool.coverage.report]
-fail_under = 80
+fail_under = 95
 show_missing = true
 skip_covered = false
 omit = [