rust: Add display_prompt to MessageOptions for session.send

Add a display_prompt field to MessageOptions that, when set, is serialized
as displayPrompt in the session.send JSON-RPC params. This allows consumers
to specify a user-facing label for the timeline that differs from the actual
prompt sent to the agent.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: devm33

rust/src/session.rs

@@ -371,6 +371,9 @@ impl Session {
         {
             params["requestHeaders"] = serde_json::to_value(headers)?;
         }
+        if let Some(display_prompt) = opts.display_prompt {
+            params["displayPrompt"] = serde_json::to_value(display_prompt)?;
+        }
         let trace_ctx = if opts.traceparent.is_some() || opts.tracestate.is_some() {
             TraceContext {
                 traceparent: opts.traceparent,

rust/src/types.rs

@@ -3107,6 +3107,8 @@ pub struct MessageOptions {
     ///
     /// Per-turn override paired with [`traceparent`](Self::traceparent).
     pub tracestate: Option<String>,
+    /// If provided, this is shown in the timeline instead of `prompt`.
+    pub display_prompt: Option<String>,
 }
 
 impl MessageOptions {
@@ -3121,6 +3123,7 @@ impl MessageOptions {
             request_headers: None,
             traceparent: None,
             tracestate: None,
+            display_prompt: None,
         }
     }
 
@@ -3181,6 +3184,12 @@ impl MessageOptions {
         self.tracestate = Some(tracestate.into());
         self
     }
+
+    /// Set the display prompt shown in the timeline instead of `prompt`.
+    pub fn with_display_prompt(mut self, display_prompt: impl Into<String>) -> Self {
+        self.display_prompt = Some(display_prompt.into());
+        self
+    }
 }
 
 impl From<&str> for MessageOptions {