Exclude tests directory from pyright type checking (#26496)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/libs/validators.py

@@ -0,0 +1,5 @@
+def validate_description_length(description: str | None) -> str | None:
+    """Validate description length."""
+    if description and len(description) > 400:
+        raise ValueError("Description cannot exceed 400 characters.")
+    return description