docs: clarify contribution testing expectations

src/docs/CONTRIBUTING.md

@@ -46,9 +46,23 @@ Please keep these project principles in mind:
 
 ```bash
 npm install
+npm test
 npm run build
 ```
 
+## Testing expectations
+
+Contributors should run the automated checks before opening a pull request:
+
+```bash
+npm test
+npm run build
+```
+
+Please add or update tests when a contribution changes behavior, parsing logic, scan logic, output shaping, or other user-visible results.
+
+If a change is difficult to cover with an automated test, include a clear reproduction case in the pull request description.
+
 ## Pull request guidelines
 
 Please try to keep pull requests focused.
@@ -58,7 +72,7 @@ A good pull request usually includes:
 - a clear description of the problem
 - the reasoning behind the change
 - before/after examples when output changes
-- tests or reproducible cases when relevant
+- tests for behavior changes, or a clear explanation when tests are not practical
 - documentation updates if user-facing behavior changed
 
 ## Commit guidance