Default Guardian v2 async classification to conversation mode (#53733)

## What changed

Use `conversation` when neither a local `async_classifier_mode` override nor a model default specifies a mode. Keep `snapshot` available as an explicit override and update the configuration documentation and schema to describe the new fallback.

## Testing

Expand configuration tests to cover all local override and model default combinations. Update app-server approval tests to exercise conversation mode without an explicit override, and explicitly select snapshot mode in tests that depend on it.

GitOrigin-RevId: a62479a3f6454bf28d589373a236d8b0ab5023b9

codex-rs/protocol/src/openai_models/guardian_v2.rs

@@ -10,8 +10,8 @@ use super::ReasoningEffort;
 #[serde(rename_all = "snake_case")]
 #[ts(rename_all = "snake_case")]
 pub enum AsyncClassifierMode {
-    #[default]
     Snapshot,
+    #[default]
     Conversation,
 }