good first issue: validate builder output type in `validate_rows`

[Blackgaurd]

Problem

validate_rows() in runtime/validator.py assumes its input is a list[dict] but never checks. A builder that returns None, a plain dict, or any non-list type will produce a confusing TypeError or AttributeError deep in iteration rather than a clear validation error.

Scope

• File: builders/server/runtime/validator.py (30 lines total)
• Change: ~3 lines added to validate_rows()
• No other files need to change

Good test to add: a unit test in builders/server/tests/runtime/test_validator.py asserting that validate_rows(None, schema) and validate_rows({}, schema) both raise ValidationError with a descriptive message.