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/thread_manager.rs

@@ -4,8 +4,7 @@ use crate::codex_thread::CodexThread;
 use crate::config::Config;
 use crate::config::ThreadStoreConfig;
 use crate::environment_selection::default_thread_environment_selections;
-use crate::environment_selection::selected_primary_environment;
-use crate::environment_selection::validate_environment_selections;
+use crate::environment_selection::resolve_environment_selections;
 use crate::file_watcher::FileWatcher;
 use crate::mcp::McpManager;
 use crate::rollout::RolloutRecorder;
@@ -433,7 +432,8 @@ impl ThreadManager {
         &self,
         environments: &[TurnEnvironmentSelection],
     ) -> CodexResult<()> {
-        validate_environment_selections(self.state.environment_manager.as_ref(), environments)
+        resolve_environment_selections(self.state.environment_manager.as_ref(), environments)
+            .map(|_| ())
     }
 
     pub fn get_models_manager(&self) -> SharedModelsManager {
@@ -1098,16 +1098,16 @@ impl ThreadManagerState {
                 threads.remove(&resumed.conversation_id);
             }
         }
-        let environment =
-            selected_primary_environment(self.environment_manager.as_ref(), &environments)?;
-        let watch_registration = match environment.as_ref() {
-            Some(environment) if !environment.is_remote() => {
+        let environment_selections =
+            resolve_environment_selections(self.environment_manager.as_ref(), &environments)?;
+        let watch_registration = match environment_selections.primary_turn_environment() {
+            Some(turn_environment) if !turn_environment.environment.is_remote() => {
                 self.skills_watcher
                     .register_config(
                         &config,
                         self.skills_manager.as_ref(),
                         self.plugins_manager.as_ref(),
-                        Some(environment.get_filesystem()),
+                        Some(turn_environment.environment.get_filesystem()),
                     )
                     .await
             }
@@ -1139,7 +1139,7 @@ impl ThreadManagerState {
             parent_rollout_thread_trace,
             user_shell_override,
             parent_trace,
-            environments,
+            environment_selections,
             analytics_events_client: self.analytics_events_client.clone(),
             thread_store: Arc::clone(&self.thread_store),
         })