docs: clarify npm test scope in contributing guide (#430)

## Summary\n- Clarify that `npm test` runs the fast suite only\n-
Document that integration tests are excluded from `npm test`\n- Add the
correct command for integration tests: `npm run test:slow`\n\n##
Why\nThe previous wording said `npm test` runs all tests in
`tests/**/*.test.js`, which is inconsistent with the actual npm script
in `package.json`. This update aligns CONTRIBUTING docs with real
behavior to reduce contributor confusion.\n\n## Local validation\n- `npm
ci --ignore-scripts`\n- `npx mocha tests/model-selection.test.js` (10
passing)

---------

Co-authored-by: haosenwang1018 <haosenwang1018@users.noreply.github.com>
Co-authored-by: Tom Dupuis <60640908+tomdps@users.noreply.github.com>

Author: haosenwang1018

CONTRIBUTING.md

@@ -122,7 +122,8 @@ zeroshot/
 npm test
 ```
 
-Runs all tests in `tests/**/*.test.js` using Mocha.
+Runs the fast test suite (unit + top-level tests) as defined in `package.json`.
+Integration tests in `tests/integration/**/*.test.js` are excluded from `npm test`; run them with `npm run test:slow`.
 
 ### Specific Test File