Serialize numeric custom reasoning effort as JSON numbers (#47590)

## What changed

Serialize custom reasoning effort values that parse as `u64` as JSON numbers in API requests. For example, `model_reasoning_effort = "64"` now sends `reasoning.effort` as `64`. Keep named values and custom values that cannot parse as `u64` as strings.

## Testing

Expand the request integration test to cover configured named effort, zero, a positive integer, an unknown name, a negative value, and an integer exceeding `u64`.

GitOrigin-RevId: 18ad03a1cd97dfa3f1a3cc49a43b3f7ca7d9b657

codex-rs/codex-api/src/common.rs

@@ -155,7 +155,10 @@ pub enum ReasoningContext {
 
 #[derive(Debug, Serialize, Clone, PartialEq)]
 pub struct Reasoning {
-    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(
+        skip_serializing_if = "Option::is_none",
+        serialize_with = "serialize_reasoning_effort"
+    )]
     pub effort: Option<ReasoningEffortConfig>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub summary: Option<ReasoningSummaryConfig>,
@@ -163,6 +166,21 @@ pub struct Reasoning {
     pub context: Option<ReasoningContext>,
 }
 
+fn serialize_reasoning_effort<S>(
+    effort: &Option<ReasoningEffortConfig>,
+    serializer: S,
+) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    if let Some(ReasoningEffortConfig::Custom(value)) = effort
+        && let Ok(value) = value.parse::<u64>()
+    {
+        return serializer.serialize_u64(value);
+    }
+    effort.serialize(serializer)
+}
+
 #[derive(Debug, Serialize, Clone, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum ReasoningSummaryDelivery {