Add strongly-typed validators to all SDKs

All SDK clients now expose typed validator objects instead of untyped JSON, making it easier to build and validate measurement validators with IDE autocomplete and compile-time checks.

• Simplified validators API schema for cleaner OpenAPI output, removing legacy string union type
• Regenerated C++, Rust, C#, and Python SDK clients with strongly-typed validator structs and builders
• Fixed C# generator to support CancellationToken in all async methods and synced SDK version to 2.3.0
• Fixed Speakeasy generation script to use correct config and synced version to 2.2.2