Centralize synchronous Guardian context preparation (#57494)

## What changed

- Retain captured root conversation sources until synchronous preparation applies request limits. Keep asynchronous snapshots limited to the bounded messages.
- Move approval-reason truncation and tool-description escaping and truncation from composition into preparation.
- Extract synchronous image preparation and admission into `guardian-context`, preserving the prompt token cap and model context-window checks.

GitOrigin-RevId: d3a864c77d8f16bd3e77df436d80c4d2738c43cc

codex-rs/guardian-context/src/section.rs

@@ -22,7 +22,7 @@ pub enum ContextSection<T = ConversationTranscriptEntry> {
         items: Vec<T>,
     },
     RootConversation {
-        items: Vec<String>,
+        content: crate::RootConversationContent,
     },
     TrustedUserAnswers {
         items: Vec<crate::TrustedUserAnswer>,