agent2: Fix tool schemas for Gemini (#36507)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent2/src/tool_schema.rs

@@ -0,0 +1,43 @@
+use language_model::LanguageModelToolSchemaFormat;
+use schemars::{
+    JsonSchema, Schema,
+    generate::SchemaSettings,
+    transform::{Transform, transform_subschemas},
+};
+
+pub(crate) fn root_schema_for<T: JsonSchema>(format: LanguageModelToolSchemaFormat) -> Schema {
+    let mut generator = match format {
+        LanguageModelToolSchemaFormat::JsonSchema => SchemaSettings::draft07().into_generator(),
+        LanguageModelToolSchemaFormat::JsonSchemaSubset => SchemaSettings::openapi3()
+            .with(|settings| {
+                settings.meta_schema = None;
+                settings.inline_subschemas = true;
+            })
+            .with_transform(ToJsonSchemaSubsetTransform)
+            .into_generator(),
+    };
+    generator.root_schema_for::<T>()
+}
+
+#[derive(Debug, Clone)]
+struct ToJsonSchemaSubsetTransform;
+
+impl Transform for ToJsonSchemaSubsetTransform {
+    fn transform(&mut self, schema: &mut Schema) {
+        // Ensure that the type field is not an array, this happens when we use
+        // Option<T>, the type will be [T, "null"].
+        if let Some(type_field) = schema.get_mut("type")
+            && let Some(types) = type_field.as_array()
+            && let Some(first_type) = types.first()
+        {
+            *type_field = first_type.clone();
+        }
+
+        // oneOf is not supported, use anyOf instead
+        if let Some(one_of) = schema.remove("oneOf") {
+            schema.insert("anyOf".to_string(), one_of);
+        }
+
+        transform_subschemas(self, schema);
+    }
+}