Carry per-input extension data to user message completion callbacks (#59455)

## What changed

Add `user_input_extension_data` to `TurnInputRequest` and carry it with each queued user input through completion, including inputs that steer an active turn. Expose the data to lifecycle contributors through `on_item_completed_with_metadata`, whose default implementation forwards to `on_item_completed` for compatibility. Keep this data out of model input and preserve the existing serialized input shape when no extension data is present.

## Testing

Add serialization round-trip coverage and extend the concurrent submission test to verify that inputs sharing a client ID retain their own metadata, report completion in the accepting turn, and omit extension data from model requests.

GitOrigin-RevId: f880cf812994a030cb1be9418280ba69e2885808

codex-rs/protocol/src/turn_input.rs

@@ -62,6 +62,9 @@ pub struct TurnInputRequest {
     /// Optional host-assigned UUID for the Responses message created from [`TurnInput::UserInput`].
     /// Ignored for all other inputs, including raw user-role Responses items.
     pub user_item_uuid: Option<uuid::Uuid>,
+    /// Host-supplied, extension-keyed data for this user input's completion.
+    /// Travels with the individual input and is never included in model input.
+    pub user_input_extension_data: BTreeMap<String, Value>,
     pub thread_settings: ThreadSettingsOverrides,
     pub start: TurnStartOptions,
     pub additional_context: BTreeMap<String, AdditionalContextEntry>,
@@ -120,6 +123,7 @@ impl TurnInputRequest {
         Self {
             input,
             user_item_uuid: None,
+            user_input_extension_data: BTreeMap::new(),
             thread_settings: ThreadSettingsOverrides::default(),
             start: TurnStartOptions::default(),
             additional_context: BTreeMap::new(),