Preserve foreign paths in managed network approvals (#39040)

## Why

Unified exec could drop network approval registration when a selected environment used a Windows working directory on a non-Windows host because the trigger path could not be converted to a host absolute path.

## What changed

- Keep network approval trigger working directories as `PathUri` values and render them with their inferred native path syntax for Guardian requests.
- Fall back to the selected turn environment only when a trigger path can be converted to a host absolute path.
- Reject selected environment working directories larger than 8 KiB at the thread-manager validation boundary.

## Testing

- Cover managed-network approval registration for a foreign Windows working directory.
- Verify that oversized working directories are rejected as invalid `thread/start` requests.

GitOrigin-RevId: 646fbf12115a35a7e5cb04b511464797bee6647b

codex-rs/app-server/tests/suite/v2/thread_start.rs

@@ -704,6 +704,44 @@ async fn thread_start_rejects_relative_environment_cwd_as_invalid_request() -> R
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_start_rejects_oversized_environment_cwd_as_invalid_request() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+    let codex_home = TempDir::new()?;
+    create_config_toml_without_approval_policy(codex_home.path(), &server.uri())?;
+
+    let mut mcp = TestAppServer::builder()
+        .with_codex_home(codex_home.path())
+        .build_initialized()
+        .await?;
+    let environment_id = mcp.auto_env_params()?.environment_id;
+
+    let request_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            environments: Some(vec![TurnEnvironmentParams {
+                environment_id,
+                cwd: serde_json::from_value(json!(format!("C:\\{}", "x".repeat(9_000))))?,
+                runtime_workspace_roots: None,
+            }]),
+            ..Default::default()
+        })
+        .await?;
+    let error: JSONRPCError = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+
+    assert_eq!(error.id, RequestId::Integer(request_id));
+    assert_eq!(error.error.code, INVALID_REQUEST_ERROR_CODE);
+    assert_eq!(
+        error.error.message,
+        "turn environment working directory exceeds the maximum size"
+    );
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_start_response_includes_loaded_instruction_sources() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;