Unify Guardian context section collection (#42076)

## What changed

- Add typed context sections for root-conversation evidence, trusted user answers, and conversation transcripts to `codex-guardian-context`.
- Use the shared section registry when building core Guardian prompts and asynchronous Guardian V2 scoring context.
- Preserve source-role labeling while delivering authorization evidence as user-role context, and fail closed when context collection fails.

## Testing

- Cover shared section ordering and role preservation for synchronous and asynchronous targets.
- Verify trusted user answers appear exactly once as user-role input in Guardian V2 approval flows.

GitOrigin-RevId: 1d5d793ffbf166077411a66e85e039596907fb7e

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

@@ -97,7 +97,7 @@ impl SectionContributor for ConversationTranscriptSection {
     }
 
     fn contribute(&self, input: &SectionInput<'_>) -> Result<Option<ContextSection>, SectionError> {
-        Ok(Some(ContextSection {
+        Ok(Some(ContextSection::ConversationTranscript {
             items: collect_transcript(input.history, input.transcript),
         }))
     }