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_thread.rs

@@ -713,7 +713,7 @@ impl CodexThread {
     fn thread_settings_update(overrides: CodexThreadSettingsOverrides) -> SessionSettingsUpdate {
         let CodexThreadSettingsOverrides {
             turn_extension_init,
-            environments,
+            environments: environment_requests,
             runtime_workspace_roots,
             profile_workspace_roots,
             approval_policy,
@@ -742,7 +742,7 @@ impl CodexThread {
                 approval_policy,
                 approvals_reviewer,
             },
-            environments,
+            environments: environment_requests,
             runtime_workspace_roots,
             profile_workspace_roots,
             sandbox_policy,