[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/thread_metadata_sync.rs

@@ -232,7 +232,7 @@ impl ThreadMetadataSync {
                         update.cwd = Some(turn_ctx.cwd.clone());
                     }
                     update.model = Some(turn_ctx.model.clone());
-                    update.reasoning_effort = turn_ctx.effort;
+                    update.reasoning_effort = turn_ctx.effort.clone();
                     update.approval_mode = Some(turn_ctx.approval_policy);
                     update.permission_profile = Some(turn_ctx.permission_profile());
                 }