Separate environment requests from runtime selections (#51221)

## What changed

Introduce `TurnEnvironmentRequest` for caller-supplied environment input and construct `TurnEnvironmentSelection` at session startup and app-server settings boundaries. Update thread startup, default environments, and inherited environment handoffs to use requests, with explicit conversions when reusing runtime selections.

Move the environment types into `protocol::environment` while retaining their `protocol` re-exports. Update test fixtures and existing tests to build request inputs separately from runtime selections.

GitOrigin-RevId: 12e7daaece95aae136afae8b055b4dbaebdffc75

codex-rs/core/src/codex_delegate.rs

@@ -11,6 +11,7 @@ use codex_protocol::protocol::Op;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::ThreadSource;
+use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_protocol::user_input::UserInput;
 use serde_json::Value;
 use std::time::Duration;
@@ -135,7 +136,11 @@ pub(crate) async fn run_codex_thread_interactive(
         inherited_exec_policy: Some(Arc::clone(&parent_session.services.exec_policy)),
         parent_rollout_thread_trace: codex_rollout_trace::ThreadTraceContext::disabled(),
         parent_trace: None,
-        environment_selections: parent_environments.to_selections(),
+        environment_requests: parent_environments
+            .to_selections()
+            .into_iter()
+            .map(TurnEnvironmentSelection::into_request)
+            .collect(),
         thread_extension_init,
         turn_extension_init: Default::default(),
         client_mcp_extensions: parent_session.services.client_mcp_extensions.clone(),