[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/config/edit.rs

@@ -225,7 +225,7 @@ impl ConfigDocument {
                 );
                 mutated |= self.write_optional_value(
                     &["model_reasoning_effort"],
-                    effort.map(|effort| value(effort.to_string())),
+                    effort.as_ref().map(|effort| value(effort.to_string())),
                 );
                 mutated
             }),