[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/memories/write/src/runtime.rs

@@ -194,7 +194,7 @@ impl MemoryStartupContext {
                 prompt,
                 &context.model_info,
                 &context.session_telemetry,
-                context.reasoning_effort,
+                context.reasoning_effort.clone(),
                 context.reasoning_summary,
                 context.service_tier.clone(),
                 context.turn_metadata_header.as_deref(),