Maintain field order when rendering class in Python (#1931)

Fix #1477 

Converting Python objects to Rust objects in the FFI code used a
`HashMap` which messes up ordering when rendering the Rust objects in
the prompt.
<!-- ELLIPSIS_HIDDEN -->


----

> [!IMPORTANT]
> Replaces `HashMap` with `IndexMap` to maintain field order in FFI
conversions and adds a test to verify this behavior.
> 
>   - **Behavior**:
> - Replaces `HashMap` with `IndexMap` in `parse_py_type.rs` to maintain
field order when converting Python objects to Rust objects.
> - Adds a new test `test_maintain_field_order` in
`prompt_renderer.test.ts` to verify field order is preserved.
>   - **Code Removal**:
> - Removes `IntoIterator` implementation for `MinijinjaBamlClass` in
`baml_value_to_jinja_value.rs`.
>   - **Misc**:
> - Adds `MaintainFieldOrder` class to `type_builder.ts` to support new
functionality.
> 
> <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 33923ee. You can
[customize](https://app.ellipsis.dev/BoundaryML/settings/summaries) this
summary. It will automatically update as commits are pushed.</sup>


<!-- ELLIPSIS_HIDDEN -->

Author: antoniosarosi

engine/baml-lib/jinja-runtime/src/baml_value_to_jinja_value.rs

@@ -1,4 +1,3 @@
-use std::collections::HashMap;
 use std::sync::Arc;
 
 use baml_types::EvaluationContext;
@@ -199,7 +198,7 @@ impl minijinja::value::StructObject for MinijinjaBamlClass {
     }
 
     fn fields(&self) -> Vec<Arc<str>> {
-        self.class.keys().into_iter().map(|k| intern(k)).collect()       
+        self.class.keys().into_iter().map(|k| intern(k)).collect()
     }
 }
 
@@ -208,18 +207,6 @@ struct MinijinjaBamlClass {
     key_to_alias: IndexMap<String, String>,
 }
 
-impl IntoIterator for MinijinjaBamlClass {
-    type Item = (String, minijinja::Value);
-    type IntoIter = std::collections::hash_map::IntoIter<String, minijinja::Value>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.class
-            .into_iter()
-            .collect::<HashMap<String, minijinja::Value>>()
-            .into_iter()
-    }
-}
-
 impl std::fmt::Display for MinijinjaBamlClass {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         let mut map = IndexMap::new();

engine/language_client_python/src/parse_py_type.rs

@@ -1,7 +1,8 @@
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 use anyhow::Result;
 use baml_types::{BamlMap, BamlValue};
+use indexmap::IndexMap;
 use pyo3::{
     exceptions::{PyRuntimeError, PyTypeError},
     prelude::{PyAnyMethods, PyTypeMethods},
@@ -51,8 +52,8 @@ impl From<Errors> for PyErr {
 
 enum MappedPyType {
     Enum(String, String),
-    Class(String, HashMap<String, PyObject>),
-    Map(HashMap<String, PyObject>),
+    Class(String, IndexMap<String, PyObject>),
+    Map(HashMap<String, PyObject>), // TODO: Does this need to maintain order?
     List(Vec<PyObject>),
     String(String),
     Int(i64),
@@ -246,11 +247,11 @@ pub fn parse_py_type(
                         }
                     })
                     .unwrap_or("<UnnamedBaseModel>".to_string());
-                let mut fields = HashMap::new();
-                // Get regular fields
+                let mut fields = IndexMap::new();
+                // Get regular fields. Maintain order, no HashMap.
                 if let Ok(model_fields) = t
                     .getattr("model_fields")?
-                    .extract::<HashMap<String, PyObject>>()
+                    .extract::<BTreeMap<String, PyObject>>()
                 {
                     for (key, _) in model_fields {
                         if let Ok(value) = any.getattr(py, key.as_str()) {

integ-tests/baml_src/test-files/prompt/field_order.baml

@@ -0,0 +1,14 @@
+class MaintainFieldOrder {
+  a string
+  b string
+  c string
+}
+
+function UseMaintainFieldOrder(input: MaintainFieldOrder) -> MaintainFieldOrder {
+  client GPT35
+  prompt #"
+    Return this value back to me: {{input}}
+  "#
+}
+
+