feat(core): add validation types

Implement ValidationContext, CheckResult, and ValidationResult models
using Pydantic with frozen (immutable) configuration.

src/veritext/core/types.py

@@ -0,0 +1,66 @@
+"""Core types for validation context and results."""
+
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class ValidationContext(BaseModel):
+    """Context for validation, containing reference text and metadata."""
+
+    model_config = ConfigDict(frozen=True)
+
+    reference: str | list[str] | None = None
+    """Reference text(s) for comparison. Required for comparison metrics."""
+
+    metadata: dict[str, Any] = Field(default_factory=dict)
+    """Additional metadata for validation (e.g., source, timestamp)."""
+
+
+class CheckResult(BaseModel):
+    """Result of a single validation check."""
+
+    model_config = ConfigDict(frozen=True)
+
+    name: str
+    """Name of the check that produced this result."""
+
+    passed: bool
+    """Whether the check passed."""
+
+    actual: Any
+    """The actual value computed by the check."""
+
+    threshold: Any | None = None
+    """The threshold the actual value was compared against, if applicable."""
+
+    message: str
+    """Human-readable description of the result."""
+
+
+class ValidationResult(BaseModel):
+    """Aggregate result of multiple validation checks."""
+
+    model_config = ConfigDict(frozen=True)
+
+    passed: bool
+    """Whether all checks passed."""
+
+    checks: list[CheckResult]
+    """Individual check results."""
+
+    @property
+    def failed_checks(self) -> list[CheckResult]:
+        """Return only the checks that failed."""
+        return [check for check in self.checks if not check.passed]
+
+    @property
+    def failure_summary(self) -> str:
+        """Return a summary of all failed checks."""
+        failed = self.failed_checks
+        if not failed:
+            return "All checks passed."
+        lines = [f"Validation failed ({len(failed)} check(s)):"]
+        for check in failed:
+            lines.append(f"  - {check.name}: {check.message}")
+        return "\n".join(lines)