Validator implementations use manual `__slots__` inconsistently with the rest of the codebase

[deyna256]

ExactMatch, Contains, Regex, and JsonSchema are defined with manual __slots__ and __init__:

class ExactMatch(Validator):
    __slots__ = ("expected", "case_sensitive")

    def __init__(self, expected: str, case_sensitive: bool = True) -> None:
        self.expected = expected
        self.case_sensitive = case_sensitive

Every other data-holding class in the codebase — Metrics, RetryConfig, Prompt, TestResult, Summary, Report — uses @dataclass(frozen=True, slots=True). The validator classes were explicitly converted to __slots__-based form in #68, moving them away from the pattern used everywhere else. There is no documented reason for this asymmetry.