[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/utils/sandbox-summary/src/config_summary.rs

@@ -25,7 +25,8 @@ pub fn create_config_summary_entries(config: &Config, model: &str) -> Vec<(&'sta
     if config.model_provider.wire_api == WireApi::Responses {
         let reasoning_effort = config
             .model_reasoning_effort
-            .map(|effort| effort.to_string());
+            .as_ref()
+            .map(std::string::ToString::to_string);
         entries.push((
             "reasoning effort",
             reasoning_effort.unwrap_or_else(|| "none".to_string()),