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/ext/guardian-v2/src/async_scorer/transcript.rs

@@ -129,6 +129,7 @@ impl TranscriptConfig {
             history: &history,
             transcript: &profile.transcript,
             root_conversation,
+            root_conversation_candidates: None,
             trusted_user_answers: &trusted_user_answers,
             planned_action,
             permissions,