[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/core/src/compact.rs

@@ -573,7 +573,7 @@ async fn drain_to_completed(
             prompt,
             &turn_context.model_info,
             &turn_context.session_telemetry,
-            turn_context.reasoning_effort,
+            turn_context.reasoning_effort.clone(),
             turn_context.reasoning_summary,
             turn_context.config.service_tier.clone(),
             turn_metadata_header,