gemini: Fix issue with builtin tool schemas (#33917)

Closes #33894

After #33635 Gemini Integration was broken because we now produce
`const` fields for enums, which are not supported.
Changing this to `openapi3` fixes the issue.

Release Notes:

- Fixed an issue where Gemini Models would not work because of
incompatible tool schemas

crates/assistant_tools/src/schema.rs

@@ -25,9 +25,7 @@ fn schema_to_json(
 fn root_schema_for<T: JsonSchema>(format: LanguageModelToolSchemaFormat) -> Schema {
     let mut generator = match format {
         LanguageModelToolSchemaFormat::JsonSchema => SchemaSettings::draft07().into_generator(),
-        // TODO: Gemini docs mention using a subset of OpenAPI 3, so this may benefit from using
-        // `SchemaSettings::openapi3()`.
-        LanguageModelToolSchemaFormat::JsonSchemaSubset => SchemaSettings::draft07()
+        LanguageModelToolSchemaFormat::JsonSchemaSubset => SchemaSettings::openapi3()
             .with(|settings| {
                 settings.meta_schema = None;
                 settings.inline_subschemas = true;