feat: add path field to ParsedCommand::Read variant (#5275)

`ParsedCommand::Read` has a `name` field that attempts to identify the
name of the file being read, but the file may not be in the `cwd` in
which the command is invoked as demonstrated by this existing unit test:


0139f6780c/codex-rs/core/src/parse_command.rs (L250-L260)

As you can see, `tui/Cargo.toml` is the relative path to the file being
read.

This PR introduces a new `path: PathBuf` field to `ParsedCommand::Read`
that attempts to capture this information. When possible, this is an
absolute path, though when relative, it should be resolved against the
`cwd` that will be used to run the command to derive the absolute path.

This should make it easier for clients to provide UI for a "read file"
event that corresponds to the command execution.

codex-rs/tui/src/chatwidget/tests.rs

@@ -1631,7 +1631,7 @@ fn status_widget_and_approval_modal_snapshot() {
     let ev = ExecApprovalRequestEvent {
         call_id: "call-approve-exec".into(),
         command: vec!["echo".into(), "hello world".into()],
-        cwd: std::path::PathBuf::from("/tmp"),
+        cwd: PathBuf::from("/tmp"),
         reason: Some(
             "this is a test reason such as one that would be produced by the model".into(),
         ),
@@ -2310,6 +2310,7 @@ fn chatwidget_exec_and_status_layout_vt100_snapshot() {
                 ParsedCommand::Read {
                     name: "diff_render.rs".into(),
                     cmd: "cat diff_render.rs".into(),
+                    path: "diff_render.rs".into(),
                 },
             ],
         }),