Prepare selected environment plumbing (#20669)

## Why
This is a prep PR in the multi-environment process-tool stack. It
separates ownership/config cleanup from the behavior change that teaches
process tools to route by selected environment, so the follow-up PR can
focus on model-facing `environment_id` behavior.

## Stack
1. https://github.com/openai/codex/pull/20646 - `EnvironmentContext`
rendering for selected environments
2. https://github.com/openai/codex/pull/20669 - selected-environment
ownership and tool config prep (this PR)
3. https://github.com/openai/codex/pull/20647 - process-tool
`environment_id` routing

## What Changed
- keep the resolved turn environment list wrapped in
`ResolvedTurnEnvironments` through `TurnContext` instead of unwrapping
it back to a raw `Vec`
- add `TurnContext::resolve_path_against` so cwd-relative path
resolution has one shared helper
- replace the old tool config boolean with `ToolEnvironmentMode::{None,
Single, Multiple}`

## Testing
- Tests not run locally; this prep refactor is covered by GitHub CI for
the stack.

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

codex-rs/core/src/unified_exec/mod_tests.rs

@@ -96,7 +96,8 @@ async fn exec_command_with_tty(
                 &request,
                 tty,
                 Box::new(NoopSpawnLifecycle),
-                turn.primary_environment()
+                turn.environments
+                    .primary()
                     .expect("turn environment")
                     .environment
                     .as_ref(),
@@ -594,7 +595,8 @@ async fn remote_exec_server_rejects_inherited_fd_launches() -> anyhow::Result<()
 
     let remote_test_env = remote_test_env().await?;
     let (_, mut turn) = make_session_and_context().await;
-    turn.environments[0].environment = Arc::new(remote_test_env.environment().clone());
+    turn.environments.turn_environments[0].environment =
+        Arc::new(remote_test_env.environment().clone());
 
     let request = test_exec_request(
         &turn,
@@ -612,7 +614,8 @@ async fn remote_exec_server_rejects_inherited_fd_launches() -> anyhow::Result<()
             Box::new(TestSpawnLifecycle {
                 inherited_fds: vec![42],
             }),
-            turn.primary_environment()
+            turn.environments
+                .primary()
                 .expect("turn environment")
                 .environment
                 .as_ref(),