[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/state/src/model/thread_metadata.rs

@@ -584,13 +584,13 @@ mod tests {
     }
 
     #[test]
-    fn thread_row_ignores_unknown_reasoning_effort_values() {
+    fn thread_row_preserves_model_defined_reasoning_effort_values() {
         let metadata = ThreadMetadata::try_from(thread_row(Some("future")))
             .expect("thread metadata should parse");
 
         assert_eq!(
             metadata,
-            expected_thread_metadata(/*reasoning_effort*/ None)
+            expected_thread_metadata(Some(ReasoningEffort::Custom("future".to_string())))
         );
     }
 }