[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/thread-store/src/in_memory.rs

@@ -369,7 +369,7 @@ fn stored_thread_from_state(
             .and_then(|metadata| metadata.model_provider.clone())
             .unwrap_or_else(|| "test".to_string()),
         model: metadata.and_then(|metadata| metadata.model.clone()),
-        reasoning_effort: metadata.and_then(|metadata| metadata.reasoning_effort),
+        reasoning_effort: metadata.and_then(|metadata| metadata.reasoning_effort.clone()),
         created_at: metadata
             .and_then(|metadata| metadata.created_at)
             .unwrap_or_else(Utc::now),