Add Schema validation endpoint

[evanvolgas]

Summary

The SDK is missing the schema validation endpoint.

Core API endpoint

• POST /schemas/validate - Validate a JSON schema without creating anything

Method to add

class SchemasResource:
    def validate(self, schema: dict) -> ValidationResult:
        """Validate a JSON schema and return any errors."""

class ValidationResult(BaseModel):
    valid: bool
    errors: list[str] | None = None

Use case

Pre-flight validation of schemas before attempting to publish contracts.