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/transcript_tests.rs

@@ -201,6 +201,7 @@ fn registered_transcript_filters_roles_and_preserves_node_repl_tool_attribution(
             history: &history,
             transcript: &config,
             root_conversation: &[],
+            root_conversation_candidates: None,
             trusted_user_answers: &[],
             planned_action: None,
             permissions: None,
@@ -290,6 +291,7 @@ fn excluded_tool_calls_still_attribute_included_results() {
             history: &history,
             transcript: &config,
             root_conversation: &[],
+            root_conversation_candidates: None,
             trusted_user_answers: &[],
             planned_action: None,
             permissions: None,
@@ -404,6 +406,7 @@ fn outputs_with_call_ids_or_explicit_names_are_retained() {
                     history: &history,
                     transcript: &config,
                     root_conversation: &[],
+                    root_conversation_candidates: None,
                     trusted_user_answers: &[],
                     planned_action: None,
                     permissions: None,
@@ -464,6 +467,7 @@ fn reused_registry_applies_current_history_sources_and_entry_limits() {
                 history: &history,
                 transcript: &config,
                 root_conversation: &[],
+                root_conversation_candidates: None,
                 trusted_user_answers: &[],
                 planned_action: None,
                 permissions: None,
@@ -506,6 +510,7 @@ fn reused_registry_applies_current_history_sources_and_entry_limits() {
                 history: &history,
                 transcript: &config,
                 root_conversation: &[],
+                root_conversation_candidates: None,
                 trusted_user_answers: &[],
                 planned_action: None,
                 permissions: None,