[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/exec/src/event_processor_with_human_output.rs

@@ -446,7 +446,8 @@ fn config_summary_entries(
             "reasoning effort",
             config
                 .model_reasoning_effort
-                .map(|effort| effort.to_string())
+                .as_ref()
+                .map(std::string::ToString::to_string)
                 .unwrap_or_else(|| "none".to_string()),
         ));
         entries.push((

codex-rs/exec/src/lib.rs

@@ -694,7 +694,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
 
     let default_cwd = config.cwd.to_path_buf();
     let default_approval_policy = config.permissions.approval_policy.value();
-    let default_effort = config.model_reasoning_effort;
+    let default_effort = config.model_reasoning_effort.clone();
 
     let (initial_operation, prompt_summary) = match (command.as_ref(), prompt, images) {
         (Some(ExecCommand::Review(review_cli)), _, _) => {
@@ -1169,7 +1169,7 @@ fn session_configured_from_thread_start_response(
         config.permissions.effective_permission_profile(),
         response.active_permission_profile.clone().map(Into::into),
         response.cwd.clone(),
-        response.reasoning_effort,
+        response.reasoning_effort.clone(),
     )
 }
 
@@ -1192,7 +1192,7 @@ fn session_configured_from_thread_resume_response(
         config.permissions.effective_permission_profile(),
         response.active_permission_profile.clone().map(Into::into),
         response.cwd.clone(),
-        response.reasoning_effort,
+        response.reasoning_effort.clone(),
     )
 }