[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/docs/codex_mcp_interface.md

@@ -68,7 +68,7 @@ Each response yields:
 - `data` - ordered list of models. A model includes:
   - `id`, `model`, `displayName`, `description`
   - `supportedReasoningEfforts` - array of objects with:
-    - `reasoningEffort` - one of `none|minimal|low|medium|high|xhigh`
+    - `reasoningEffort` - a string value advertised by the model; common values are `none|minimal|low|medium|high|xhigh`
     - `description` - human-friendly label for the effort
   - `defaultReasoningEffort` - suggested effort for the UI
   - `inputModalities` - accepted input types for the model