[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/tui/src/chatwidget/session_flow.rs

@@ -79,7 +79,7 @@ impl ChatWidget {
         let default_model = session.model.clone();
         self.current_collaboration_mode = self.current_collaboration_mode.with_updates(
             Some(default_model.clone()),
-            Some(session.reasoning_effort),
+            Some(session.reasoning_effort.clone()),
             /*developer_instructions*/ None,
         );
         match session.collaboration_mode.as_deref() {
@@ -93,7 +93,7 @@ impl ChatWidget {
                     Some(&default_model),
                 );
                 if let Some(mask) = self.active_collaboration_mask.as_mut() {
-                    mask.reasoning_effort = Some(session.reasoning_effort);
+                    mask.reasoning_effort = Some(session.reasoning_effort.clone());
                 }
                 self.update_collaboration_mode_indicator();
                 self.refresh_plan_mode_nudge();