Centralize turn environment selection state (#38461)

## What changed

- Store `TurnEnvironmentSelection` directly on each resolved `TurnEnvironment`
  instead of copying its environment ID, working directory, and workspace roots
  into separate fields.
- Update environment resolution, tool execution, approvals, and related tests to
  access selection state through the retained value.

GitOrigin-RevId: ab723d21e112b8af5bcaf1f772a5e376553312bc

codex-rs/core/src/session/mod.rs

@@ -3160,7 +3160,7 @@ impl Session {
                 .turn_environments()
                 .map(|environment| {
                     (
-                        environment.environment_id.clone(),
+                        environment.selection.environment_id.clone(),
                         turn_context.file_system_sandbox_context(
                             /*additional_permissions*/ None,
                             environment,