Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

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

codex-rs/core/tests/suite/items.rs

@@ -85,6 +85,7 @@ async fn user_message_item_is_emitted() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![expected_input.clone()],
             final_output_json_schema: None,
             responsesapi_client_metadata: None,
@@ -139,6 +140,7 @@ async fn assistant_message_item_is_emitted() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please summarize results".into(),
                 text_elements: Vec::new(),
@@ -198,6 +200,7 @@ async fn reasoning_item_is_emitted() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "explain your reasoning".into(),
                 text_elements: Vec::new(),
@@ -258,6 +261,7 @@ async fn web_search_item_is_emitted() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "find the weather".into(),
                 text_elements: Vec::new(),
@@ -323,6 +327,7 @@ async fn image_generation_call_event_is_emitted() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "generate a tiny blue square".into(),
                 text_elements: Vec::new(),
@@ -386,6 +391,7 @@ async fn image_generation_call_event_is_emitted_when_image_save_fails() -> anyho
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "generate an image".into(),
                 text_elements: Vec::new(),
@@ -440,6 +446,7 @@ async fn agent_message_content_delta_has_item_metadata() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please stream text".into(),
                 text_elements: Vec::new(),
@@ -523,6 +530,7 @@ async fn plan_mode_emits_plan_item_from_proposed_plan_block() -> anyhow::Result<
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please plan".into(),
                 text_elements: Vec::new(),
@@ -600,6 +608,7 @@ async fn plan_mode_strips_plan_from_agent_messages() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please plan".into(),
                 text_elements: Vec::new(),
@@ -709,6 +718,7 @@ async fn plan_mode_streaming_citations_are_stripped_across_added_deltas_and_done
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please plan with citations".into(),
                 text_elements: Vec::new(),
@@ -896,6 +906,7 @@ async fn plan_mode_streaming_proposed_plan_tag_split_across_added_and_delta_is_p
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please plan".into(),
                 text_elements: Vec::new(),
@@ -1010,6 +1021,7 @@ async fn plan_mode_handles_missing_plan_close_tag() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please plan".into(),
                 text_elements: Vec::new(),
@@ -1088,6 +1100,7 @@ async fn reasoning_content_delta_has_item_metadata() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "reason through it".into(),
                 text_elements: Vec::new(),
@@ -1148,6 +1161,7 @@ async fn reasoning_raw_content_delta_respects_flag() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "show raw reasoning".into(),
                 text_elements: Vec::new(),