Use selected turn environments for runtime context (#20281)

## Summary
- make selected turn environments the source of truth for session
runtime cwd and MCP runtime environment selection
- keep local/no-selection fallback behavior intact
- add coverage for duplicate selected environments, cwd resolution, and
MCP runtime environment selection

## Validation
- git diff --check
- rustfmt was run on touched Rust files during the implementation
workflow

CI should provide the full Bazel/test signal.

---------

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

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

@@ -96,7 +96,10 @@ async fn exec_command_with_tty(
                 &request,
                 tty,
                 Box::new(NoopSpawnLifecycle),
-                turn.environment.as_ref().expect("turn environment"),
+                turn.primary_environment()
+                    .expect("turn environment")
+                    .environment
+                    .as_ref(),
             )
             .await?,
     );
@@ -591,7 +594,7 @@ 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.environment = Some(Arc::new(remote_test_env.environment().clone()));
+    turn.environments[0].environment = Arc::new(remote_test_env.environment().clone());
 
     let request = test_exec_request(
         &turn,
@@ -609,7 +612,10 @@ async fn remote_exec_server_rejects_inherited_fd_launches() -> anyhow::Result<()
             Box::new(TestSpawnLifecycle {
                 inherited_fds: vec![42],
             }),
-            turn.environment.as_ref().expect("turn environment"),
+            turn.primary_environment()
+                .expect("turn environment")
+                .environment
+                .as_ref(),
         )
         .await
         .expect_err("expected inherited fd rejection");