Preserve null developer instructions (#16976)

Preserve explicit null developer-instruction overrides across app-server
resume and fork flows.

codex-rs/core/src/config/mod.rs

@@ -248,6 +248,10 @@ pub struct Config {
     /// Developer instructions override injected as a separate message.
     pub developer_instructions: Option<String>,
 
+    /// Explicit developer instructions override, preserving `null` as distinct
+    /// from a missing override.
+    pub developer_instructions_override: Option<Option<String>>,
+
     /// Guardian-specific developer instructions override from requirements.toml.
     pub guardian_developer_instructions: Option<String>,
 
@@ -1761,6 +1765,7 @@ impl Config {
         let file_base_instructions =
             Self::try_read_non_empty_file(model_instructions_path, "model instructions file")?;
         let base_instructions = base_instructions.or_else(|| file_base_instructions.map(Some));
+        let developer_instructions_override = developer_instructions.clone();
         let developer_instructions =
             developer_instructions.unwrap_or_else(|| cfg.developer_instructions.clone());
         let include_permissions_instructions = config_profile
@@ -1945,6 +1950,7 @@ impl Config {
             base_instructions,
             personality,
             developer_instructions,
+            developer_instructions_override,
             compact_prompt,
             commit_attribution,
             include_permissions_instructions,