Resolve rustapi_validate/core paths dynamically

Use proc_macro_crate to detect how validation/core/async_trait crates are referenced and emit the correct paths for generated code. The macros crate now computes validate_path, core_path, and async_trait_path and threads them into generation functions (generate_rule_validation, generate_async_rule_validation and derive_validate) so the Validate/AsyncValidate/Validatable impls work whether the consumer depends on rustapi-rs or the underlying crates directly. Added rustapi_validate to rustapi-core::__private and re-exported async_trait in rustapi-rs::__private to support those resolution strategies.

Author: Tuntii

crates/rustapi-core/src/lib.rs

@@ -93,6 +93,7 @@ pub mod __private {
     pub use crate::auto_schema::AUTO_SCHEMAS;
     pub use linkme;
     pub use rustapi_openapi;
+    pub use rustapi_validate;
 }
 
 // Public API