Fix validation to reject field-level assertions in test blocks (#2259)

## Summary
Fixes #1252 by adding validation to reject field-level `@assert` and
`@check` attributes on test blocks. These attributes were previously
accepted by the parser but silently ignored at runtime, causing
confusion.

## Problem
- BAML tests incorrectly accept `@assert` (single @) syntax without
warnings
- Field-level assertions on test fields are silently ignored at runtime
- Only block-level `@@assert` (double @) assertions actually work for
tests
- This leads to users writing non-functional tests that appear to work

## Solution
Added semantic validation in the test validator to:
1. Check all fields in test blocks for `@assert` or `@check` attributes
2. Emit clear error messages directing users to use `@@assert` or
`@@check` at the block level instead
3. Point to the exact location of the invalid attribute

## Changes
- **Modified**:
`engine/baml-lib/baml-core/src/validate/validation_pipeline/validations/tests.rs`
  - Added validation logic to detect field-level assertion attributes
  - Provides helpful error messages with the correct syntax
- **Added**: Test files to verify the validation works correctly
  - `field_level_assertions_v2.baml` - validates @Assert rejection
  - `field_level_check.baml` - validates @check rejection

## Testing
- ✅ All existing tests pass
- ✅ New validation tests pass with expected error messages
- ✅ No clippy warnings
- ✅ Error messages point to correct locations

## Example Error
```
error: @Assert is not allowed on test fields. Use @@Assert at the test block level instead.
  -->  functions_v2/tests/field_level_assertions_v2.baml:9
   |
 8 | test MyTest {
 9 |   functions [TestFunction] @Assert({{ true }})
   |
```

🤖 Generated with [Claude Code](https://claude.ai/code)
<!-- ELLIPSIS_HIDDEN -->

----

> [!IMPORTANT]
> Adds validation to reject field-level `@assert` and `@check`
attributes in test blocks, providing clear error messages and directing
users to use block-level assertions.
> 
>   - **Behavior**:
> - Adds validation to reject field-level `@assert` and `@check`
attributes in test blocks, directing users to use `@@assert` or
`@@check` at the block level.
> - Provides error messages pointing to the exact location of invalid
attributes.
>   - **Validation Logic**:
> - Modified `validate()` in `tests.rs` to include logic for detecting
field-level assertion attributes.
>   - **Testing**:
> - Added `field_level_assertions_v2.baml` and `field_level_check.baml`
to test validation of `@assert` and `@check` rejection.
>     - All existing and new tests pass, ensuring no regression.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 86029cc. You can
[customize](https://app.ellipsis.dev/BoundaryML/settings/summaries) this
summary. It will automatically update as commits are pushed.</sup>

<!-- ELLIPSIS_HIDDEN -->

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: aaronvg <aaron@boundaryml.com>

Author: 3 people

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

@@ -1,4 +1,5 @@
 use baml_types::{Constraint, ConstraintLevel};
+use internal_baml_ast::ast::WithName;
 use internal_baml_diagnostics::{DatamodelError, DatamodelWarning, Span};
 use internal_baml_jinja_types::{validate_expression, JinjaContext, PredefinedTypes, Type};
 
@@ -7,6 +8,23 @@ use crate::validate::validation_pipeline::context::Context;
 pub(super) fn validate(ctx: &mut Context<'_>) {
     let tests = ctx.db.walk_test_cases().collect::<Vec<_>>();
     tests.iter().for_each(|walker| {
+        // Validate that test fields don't have @assert or @check attributes
+        let test_ast = walker.ast_node();
+        for (_field_id, field) in test_ast.iter_fields() {
+            for attr in &field.attributes {
+                if attr.name.name() == "assert" || attr.name.name() == "check" {
+                    ctx.push_error(DatamodelError::new_validation_error(
+                        &format!(
+                            "@{} is not allowed on test fields. Use @@{} at the test block level instead.",
+                            attr.name.name(),
+                            attr.name.name()
+                        ),
+                        attr.span.clone(),
+                    ));
+                }
+            }
+        }
+
         let constraints = &walker.test_case().constraints;
         let args = &walker.test_case().args;
         let mut check_names: Vec<String> = Vec::new();

engine/baml-lib/baml/tests/validation_files/functions_v2/tests/field_level_assertions_v2.baml

@@ -0,0 +1,20 @@
+// Test that field-level assertions are not allowed on test block fields
+
+function TestFunction(input: string) -> string {
+  client "openai/gpt-4"
+  prompt #"Test function"#
+}
+
+test MyTest {
+  functions [TestFunction] @assert({{ true }})
+  args {
+    input "hello"
+  }
+}
+
+// error: Error validating: @assert is not allowed on test fields. Use @@assert at the test block level instead.
+//   -->  functions_v2/tests/field_level_assertions_v2.baml:9
+//    | 
+//  8 | test MyTest {
+//  9 |   functions [TestFunction] @assert({{ true }})
+//    | 

engine/baml-lib/baml/tests/validation_files/functions_v2/tests/field_level_check.baml

@@ -0,0 +1,20 @@
+// Test that field-level @check is not allowed on test block fields
+
+function TestFunction(input: string) -> string {
+  client "openai/gpt-4"
+  prompt #"Test function"#
+}
+
+test MyTest {
+  functions [TestFunction]
+  args {
+    input "hello"
+  } @check(args_check, {{ true }})
+}
+
+// error: Error validating: @check is not allowed on test fields. Use @@check at the test block level instead.
+//   -->  functions_v2/tests/field_level_check.baml:12
+//    | 
+// 11 |     input "hello"
+// 12 |   } @check(args_check, {{ true }})
+//    |