[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-protocol/src/protocol/thread_history.rs

@@ -621,7 +621,7 @@ impl ThreadHistoryBuilder {
             receiver_thread_ids: Vec::new(),
             prompt: Some(payload.prompt.clone()),
             model: Some(payload.model.clone()),
-            reasoning_effort: Some(payload.reasoning_effort),
+            reasoning_effort: Some(payload.reasoning_effort.clone()),
             agents_states: HashMap::new(),
         };
         self.upsert_item_in_current_turn(item);
@@ -656,7 +656,7 @@ impl ThreadHistoryBuilder {
             receiver_thread_ids,
             prompt: Some(payload.prompt.clone()),
             model: Some(payload.model.clone()),
-            reasoning_effort: Some(payload.reasoning_effort),
+            reasoning_effort: Some(payload.reasoning_effort.clone()),
             agents_states,
         });
     }