Support typed content in contextual prompt fragments (#54776)

## Why

Contextual fragments and extension-owned world-state sections currently return strings, limiting them to text content.

## What changed

- Return `ContentItem` from fragment bodies and rendering, preserving classification and attribution. Apply text markers only to `InputText`; pass other content through unchanged.
- Allow extension prompt and world-state contributions to carry typed content, and preserve media in context-window hints.
- Hash non-text world-state fragments by their serialized content and role while retaining the existing text hashing behavior.
- Add shared content-to-tool-output conversion that preserves image references, image detail, and audio.

## Testing

Add coverage for typed world-state hashing and preservation of image content and attribution. Expand code-mode output tests to cover input text, output text, file-backed and inline images, and audio.

GitOrigin-RevId: 2ed00d466a3f169b151202f3eef71e667ea64be1

codex-rs/core/src/session_prefix.rs

@@ -33,7 +33,12 @@ pub(crate) fn format_inter_agent_completion_message(
         AgentStatus::NotFound => "Agent was not found.".to_string(),
         AgentStatus::PendingInit | AgentStatus::Running | AgentStatus::Interrupted => return None,
     };
-    Some(InterAgentCompletionMessage::new(task_name, sender, payload).render())
+    Some(
+        InterAgentCompletionMessage::new(task_name, sender, payload)
+            .render()
+            .into_text()
+            .unwrap_or_default(),
+    )
 }
 
 pub(crate) fn format_guardian_interruption_message(
@@ -43,7 +48,10 @@ pub(crate) fn format_guardian_interruption_message(
 ) -> String {
     let error = truncate_text(error, TruncationPolicy::Tokens(ERROR_MAX_TOKENS));
     let payload = format!("Agent interrupted by Guardian: {error}\n\n{GUARDIAN_NEXT_ACTION}");
-    InterAgentCompletionMessage::new(task_name, sender, payload).render()
+    InterAgentCompletionMessage::new(task_name, sender, payload)
+        .render()
+        .into_text()
+        .unwrap_or_default()
 }
 
 #[cfg(test)]