[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/tests/common/models_cache.rs

@@ -18,7 +18,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         slug: preset.id.clone(),
         display_name: preset.display_name.clone(),
         description: Some(preset.description.clone()),
-        default_reasoning_level: Some(preset.default_reasoning_effort),
+        default_reasoning_level: Some(preset.default_reasoning_effort.clone()),
         supported_reasoning_levels: preset.supported_reasoning_efforts.clone(),
         shell_type: ConfigShellToolType::ShellCommand,
         visibility: if preset.show_in_picker {