[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/tui/src/history_cell/session.rs

@@ -149,7 +149,7 @@ pub(crate) fn new_session_info(
     // Header box rendered as history (so it appears at the very top)
     let header = SessionHeaderHistoryCell::new(
         session.model.clone(),
-        session.reasoning_effort,
+        session.reasoning_effort.clone(),
         show_fast_status,
         config.cwd.to_path_buf(),
         CODEX_CLI_VERSION,
@@ -317,15 +317,10 @@ impl SessionHeaderHistoryCell {
         formatted
     }
 
-    fn reasoning_label(&self) -> Option<&'static str> {
-        self.reasoning_effort.map(|effort| match effort {
-            ReasoningEffortConfig::Minimal => "minimal",
-            ReasoningEffortConfig::Low => "low",
-            ReasoningEffortConfig::Medium => "medium",
-            ReasoningEffortConfig::High => "high",
-            ReasoningEffortConfig::XHigh => "xhigh",
-            ReasoningEffortConfig::None => "none",
-        })
+    fn reasoning_label(&self) -> Option<&str> {
+        self.reasoning_effort
+            .as_ref()
+            .map(ReasoningEffortConfig::as_str)
     }
 }
 
@@ -368,7 +363,7 @@ impl HistoryCell for SessionHeaderHistoryCell {
             ];
             if let Some(reasoning) = reasoning_label {
                 spans.push(Span::from(" "));
-                spans.push(Span::from(reasoning));
+                spans.push(Span::from(reasoning.to_owned()));
             }
             if self.show_fast_status {
                 spans.push("   ".into());