[codex] Support model-defined reasoning efforts (#26444)

## Summary
- accept non-empty model-defined reasoning effort values while
preserving built-in effort behavior
- propagate the non-Copy effort type through core, app-server, TUI,
telemetry, and persistence call sites
- preserve string wire encoding and expose an open-string schema for
clients
- update model selection and shortcut behavior for model-advertised
effort values

## Root cause
`ReasoningEffort` gained a string-backed custom variant, so it could no
longer implement `Copy` or rely on derived closed-enum serialization.
Existing consumers still moved effort values from shared references and
assumed a fixed built-in value set.

## Validation
- `just fmt`
- Local tests and compilation were not run per request; relying on CI.

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -158,7 +158,7 @@ fn merge_persisted_resume_metadata(
     typesafe_overrides.model = persisted_metadata.model.clone();
     typesafe_overrides.model_provider = Some(persisted_metadata.model_provider.clone());
 
-    if let Some(reasoning_effort) = persisted_metadata.reasoning_effort {
+    if let Some(reasoning_effort) = persisted_metadata.reasoning_effort.as_ref() {
         request_overrides.get_or_insert_with(HashMap::new).insert(
             "model_reasoning_effort".to_string(),
             serde_json::Value::String(reasoning_effort.to_string()),