Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-experimental-api-macros/src/lib.rs

@@ -261,11 +261,8 @@ fn presence_expr_for_access(
     access: proc_macro2::TokenStream,
     ty: &Type,
 ) -> proc_macro2::TokenStream {
-    if let Some(inner) = option_inner(ty) {
-        let inner_expr = presence_expr_for_ref(quote!(value), inner);
-        return quote! {
-            #access.as_ref().is_some_and(|value| #inner_expr)
-        };
+    if option_inner(ty).is_some() {
+        return quote! { #access.is_some() };
     }
     if is_vec_like(ty) || is_map_like(ty) {
         return quote! { !#access.is_empty() };
@@ -276,22 +273,6 @@ fn presence_expr_for_access(
     quote! { true }
 }
 
-fn presence_expr_for_ref(access: proc_macro2::TokenStream, ty: &Type) -> proc_macro2::TokenStream {
-    if let Some(inner) = option_inner(ty) {
-        let inner_expr = presence_expr_for_ref(quote!(value), inner);
-        return quote! {
-            #access.as_ref().is_some_and(|value| #inner_expr)
-        };
-    }
-    if is_vec_like(ty) || is_map_like(ty) {
-        return quote! { !#access.is_empty() };
-    }
-    if is_bool(ty) {
-        return quote! { *#access };
-    }
-    quote! { true }
-}
-
 fn option_inner(ty: &Type) -> Option<&Type> {
     let Type::Path(type_path) = ty else {
         return None;