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/session/session.rs

@@ -968,7 +968,7 @@ impl Session {
                     "unknown stored MCP environment id",
                 ))
             })?
-            .primary_turn_environment()
+            .primary()
             .cloned();
             let mcp_runtime_environment = match turn_environment {
                 Some(turn_environment) => McpRuntimeEnvironment::new(