sandboxing: migrate cwd inputs to PathUri (#27816)

## Why

Sandbox cwd values can cross app-server and exec-server host boundaries.
They should retain URI semantics until the receiving host validates them
instead of being interpreted early as native paths.

## What

- Carry `PathUri` through filesystem sandbox contexts, sandbox commands,
and transform inputs.
- Convert command and policy cwd once in `SandboxManager::transform`,
then keep launch requests native.
- Preserve sandbox cwd over remote filesystem transport and reject
non-native URIs without fallback.
- Cache paired native/URI turn-environment cwd values during migration,
with immutable access to keep them synchronized.
- Extend existing protocol, forwarding, transform, and core runtime
tests.

codex-rs/core/src/environment_selection.rs

@@ -58,7 +58,7 @@ impl ResolvedTurnEnvironments {
             return None;
         };
 
-        (!environment.environment.is_remote()).then_some(&environment.cwd)
+        (!environment.environment.is_remote()).then_some(environment.cwd())
     }
 }
 
@@ -96,12 +96,12 @@ pub(crate) async fn resolve_environment_selections(
                 None
             }
         };
-        turn_environments.push(TurnEnvironment {
+        turn_environments.push(TurnEnvironment::new(
             environment_id,
             environment,
-            cwd: selected_environment.cwd.clone(),
+            selected_environment.cwd.clone(),
             shell,
-        });
+        )?);
     }
     Ok(ResolvedTurnEnvironments { turn_environments })
 }
@@ -270,22 +270,26 @@ url = "ws://127.0.0.1:8765"
                 .expect("remote environment"),
         );
         let remote = ResolvedTurnEnvironments {
-            turn_environments: vec![TurnEnvironment {
-                environment_id: REMOTE_ENVIRONMENT_ID.to_string(),
-                environment: remote_environment.clone(),
-                cwd: cwd.clone(),
-                shell: None,
-            }],
+            turn_environments: vec![
+                TurnEnvironment::new(
+                    REMOTE_ENVIRONMENT_ID.to_string(),
+                    remote_environment.clone(),
+                    cwd.clone(),
+                    /*shell*/ None,
+                )
+                .expect("remote cwd URI"),
+            ],
         };
         let multiple = ResolvedTurnEnvironments {
             turn_environments: vec![
                 local.primary().expect("local environment").clone(),
-                TurnEnvironment {
-                    environment_id: REMOTE_ENVIRONMENT_ID.to_string(),
-                    environment: remote_environment,
-                    cwd: cwd.clone(),
-                    shell: None,
-                },
+                TurnEnvironment::new(
+                    REMOTE_ENVIRONMENT_ID.to_string(),
+                    remote_environment,
+                    cwd.clone(),
+                    /*shell*/ None,
+                )
+                .expect("remote cwd URI"),
             ],
         };