Preserve null developer instructions (#16976)

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

codex-rs/core/src/codex.rs

@@ -615,6 +615,14 @@ impl Codex {
             dynamic_tools
         };
 
+        let developer_instructions_override = config
+            .developer_instructions_override
+            .clone()
+            .or_else(|| conversation_history.get_developer_instructions());
+        let developer_instructions = developer_instructions_override
+            .clone()
+            .unwrap_or_else(|| config.developer_instructions.clone());
+
         // TODO (aibrahim): Consolidate config.model and config.model_reasoning_effort into config.collaboration_mode
         // to avoid extracting these fields separately and constructing CollaborationMode here.
         let collaboration_mode = CollaborationMode {
@@ -630,7 +638,8 @@ impl Codex {
             collaboration_mode,
             model_reasoning_summary: config.model_reasoning_summary,
             service_tier: config.service_tier,
-            developer_instructions: config.developer_instructions.clone(),
+            developer_instructions,
+            developer_instructions_override,
             user_instructions,
             personality: config.personality,
             base_instructions,
@@ -1095,6 +1104,10 @@ pub(crate) struct SessionConfiguration {
     /// Developer instructions that supplement the base instructions.
     developer_instructions: Option<String>,
 
+    /// Explicit developer instructions override, preserving `null` as distinct
+    /// from a missing override.
+    developer_instructions_override: Option<Option<String>>,
+
     /// Model instructions that are appended to the base instructions.
     user_instructions: Option<String>,
 
@@ -1545,6 +1558,9 @@ impl Session {
                             .base_instructions
                             .clone()
                             .map(|text| BaseInstructions { text }),
+                        session_configuration
+                            .developer_instructions_override
+                            .clone(),
                         session_configuration.dynamic_tools.clone(),
                         if session_configuration.persist_extended_history {
                             EventPersistenceMode::Extended