Preserve executor paths in read command actions (#36223)

## Why

Read command actions were omitted when a selected environment used a path
convention foreign to the app-server host. Clients need these actions to refer
to files in the executor's filesystem, not paths interpreted on the host.

## What changed

- Resolve read paths against the executor's `PathUri` and serialize them using
  the executor's native POSIX, Windows drive, or UNC convention.
- Accept same-drive Windows-relative paths while continuing to reject paths
  that require another drive's executor-local current directory.
- Document the path convention used by `commandExecution` notifications.

## Testing

Cover native and cross-platform path serialization, Windows drive-relative
joins, and selected-environment `item/started` and `item/completed`
notifications.

GitOrigin-RevId: 3c5a9bb1caafd35e04a0a0cceab91fee590372b9

codex-rs/analytics/src/analytics_client_tests.rs

@@ -2449,7 +2449,7 @@ async fn item_lifecycle_notifications_publish_command_execution_event() {
                             CommandAction::Read {
                                 command: "cat README.md".to_string(),
                                 name: "README.md".to_string(),
-                                path: test_path_buf("/tmp/README.md").abs(),
+                                path: test_path_buf("/tmp/README.md").abs().into(),
                             },
                             CommandAction::ListFiles {
                                 command: "ls".to_string(),