Add ItemStarted/ItemCompleted events for UserInputItem (#5306)

Adds a new ItemStarted event and delivers UserMessage as the first item
type (more to come).


Renames `InputItem` to `UserInput` considering we're using the `Item`
suffix for actual items.

codex-rs/protocol/src/user_input.rs

@@ -0,0 +1,24 @@
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use ts_rs::TS;
+
+/// User input
+#[non_exhaustive]
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS, JsonSchema)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum UserInput {
+    Text {
+        text: String,
+    },
+    /// Pre‑encoded data: URI image.
+    Image {
+        image_url: String,
+    },
+
+    /// Local image path provided by the user.  This will be converted to an
+    /// `Image` variant (base64 data URL) during request serialization.
+    LocalImage {
+        path: std::path::PathBuf,
+    },
+}