Pass TurnContext around instead of sub_id (#5421)

Today `sub_id` is an ID of a single incoming Codex Op submition. We then
associate all events triggered by this operation using the same
`sub_id`.

At the same time we are also creating a TurnContext per submission and
we'd like to start associating some events (item added/item completed)
with an entire turn instead of just the operation that started it.

Using turn context when sending events give us flexibility to change
notification scheme.

codex-rs/core/src/tools/context.rs

@@ -24,7 +24,6 @@ pub struct ToolInvocation {
     pub session: Arc<Session>,
     pub turn: Arc<TurnContext>,
     pub tracker: SharedTurnDiffTracker,
-    pub sub_id: String,
     pub call_id: String,
     pub tool_name: String,
     pub payload: ToolPayload,
@@ -234,7 +233,7 @@ mod tests {
 #[derive(Clone, Debug)]
 #[allow(dead_code)]
 pub(crate) struct ExecCommandContext {
-    pub(crate) sub_id: String,
+    pub(crate) turn: Arc<TurnContext>,
     pub(crate) call_id: String,
     pub(crate) command_for_display: Vec<String>,
     pub(crate) cwd: PathBuf,