improve error message for asserts and checks (#1975)

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> [!IMPORTANT]
> Improved error messages for `check` and `assert` constraints in
`validate_type_constraints()` in `types.rs`.
> 
>   - **Error Messages**:
> - Improved error message for `check` constraints in
`validate_type_constraints()` in `types.rs` to specify that a valid
identifier is required.
> - Enhanced error message for `check` and `assert` constraints to
clarify the expected format, including examples.
> 
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Author: imalsogreg

engine/baml-lib/baml-core/src/validate/validation_pipeline/validations/types.rs

@@ -243,16 +243,24 @@ fn validate_type_constraints(ctx: &mut Context<'_>, field_type: &FieldType) {
 
                 if name.to_string() == "check" {
                     ctx.push_error(DatamodelError::new_validation_error(
-                        "Check constraints must have a name.",
+                        r#"
+Check constraints must have a valid identifier. e.g.:
+@check( valid_name_length, {{ this|length > 0 }} )
+                        "#,
                         span.clone(),
                     ))
                 }
             }
             _ => {
                 ctx.push_error(DatamodelError::new_validation_error(
-                        "A constraint must have one Jinja argument such as {{ expr }}, and optionally one String label",
-                        span.clone()
-                    ));
+                    r#"
+A check or assert has an optional identifier and a Jinja argument. e.g.:
+@check( valid_name_length, {{ this|length > 0 }} )
+@assert( {{ this|length > 0 }} )
+@assert( valid_name_length, {{ this|length > 0 }} )
+                        "#,
+                    span.clone(),
+                ));
             }
         }
     }