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/codex_delegate.rs

@@ -30,7 +30,6 @@ use tokio::time::timeout;
 use tokio_util::sync::CancellationToken;
 
 use crate::config::Config;
-use crate::environment_selection::ResolvedTurnEnvironments;
 use crate::guardian::GuardianApprovalRequest;
 use crate::guardian::new_guardian_review_id;
 use crate::guardian::routes_approval_to_guardian;
@@ -94,9 +93,7 @@ pub(crate) async fn run_codex_thread_interactive(
         inherited_exec_policy: Some(Arc::clone(&parent_session.services.exec_policy)),
         parent_rollout_thread_trace: codex_rollout_trace::ThreadTraceContext::disabled(),
         parent_trace: None,
-        environment_selections: ResolvedTurnEnvironments {
-            turn_environments: parent_ctx.environments.clone(),
-        },
+        environment_selections: parent_ctx.environments.clone(),
         analytics_events_client: Some(parent_session.services.analytics_events_client.clone()),
         thread_store: Arc::clone(&parent_session.services.thread_store),
     }))