[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/session/turn_context.rs

@@ -136,7 +136,8 @@ impl TurnContext {
     pub(crate) fn effective_reasoning_effort(&self) -> Option<ReasoningEffortConfig> {
         if self.model_info.supports_reasoning_summaries {
             self.reasoning_effort
-                .or(self.model_info.default_reasoning_level)
+                .clone()
+                .or_else(|| self.model_info.default_reasoning_level.clone())
         } else {
             None
         }
@@ -196,28 +197,29 @@ impl TurnContext {
         let supported_reasoning_levels = model_info
             .supported_reasoning_levels
             .iter()
-            .map(|preset| preset.effort)
+            .map(|preset| preset.effort.clone())
             .collect::<Vec<_>>();
-        let reasoning_effort = if let Some(current_reasoning_effort) = self.reasoning_effort {
+        let reasoning_effort = if let Some(current_reasoning_effort) = self.reasoning_effort.clone()
+        {
             if supported_reasoning_levels.contains(&current_reasoning_effort) {
                 Some(current_reasoning_effort)
             } else {
                 supported_reasoning_levels
                     .get(supported_reasoning_levels.len().saturating_sub(1) / 2)
-                    .copied()
-                    .or(model_info.default_reasoning_level)
+                    .cloned()
+                    .or_else(|| model_info.default_reasoning_level.clone())
             }
         } else {
             supported_reasoning_levels
                 .get(supported_reasoning_levels.len().saturating_sub(1) / 2)
-                .copied()
-                .or(model_info.default_reasoning_level)
+                .cloned()
+                .or_else(|| model_info.default_reasoning_level.clone())
         };
-        config.model_reasoning_effort = reasoning_effort;
+        config.model_reasoning_effort = reasoning_effort.clone();
 
         let collaboration_mode = self.collaboration_mode.with_updates(
             Some(model.clone()),
-            Some(reasoning_effort),
+            Some(reasoning_effort.clone()),
             /*developer_instructions*/ None,
         );
         let features = self.features.clone();
@@ -363,7 +365,7 @@ impl TurnContext {
             collaboration_mode: Some(self.collaboration_mode.clone()),
             multi_agent_version: Some(self.multi_agent_version),
             realtime_active: Some(self.realtime_active),
-            effort: self.reasoning_effort,
+            effort: self.reasoning_effort.clone(),
             summary: ReasoningSummaryConfig::Auto,
         }
     }