app-server: treat null mode developer instructions as built-in defaults (#10983)

## Summary
- make `turn/start` normalize
`collaborationMode.settings.developer_instructions: null` to the
built-in instructions for the selected mode
- prevent app-server clients from accidentally clearing mode-switch
developer instructions by sending `null`
- document this behavior in the v2 protocol and app-server docs

## What changed
- `codex-rs/app-server/src/codex_message_processor.rs`
  - added a small `normalize_turn_start_collaboration_mode` helper
  - in `turn_start`, apply normalization before `OverrideTurnContext`
- `codex-rs/app-server/tests/suite/v2/turn_start.rs`
- extended `turn_start_accepts_collaboration_mode_override_v2` to assert
the outgoing request includes default-mode instruction text when the
client sends `developer_instructions: null`
- `codex-rs/app-server-protocol/src/protocol/v2.rs`
- clarified `TurnStartParams.collaboration_mode` docs:
`settings.developer_instructions: null` means use built-in mode
instructions
- regenerated schema fixture:
- `codex-rs/app-server-protocol/schema/typescript/v2/TurnStartParams.ts`
- docs:
  - `codex-rs/app-server/README.md`
  - `codex-rs/docs/codex_mcp_interface.md`

codex-rs/app-server/src/codex_message_processor.rs

@@ -206,6 +206,7 @@ use codex_login::ServerOptions as LoginServerOptions;
 use codex_login::ShutdownHandle;
 use codex_login::run_login_server;
 use codex_protocol::ThreadId;
+use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::WindowsSandboxLevel;
@@ -397,6 +398,27 @@ impl CodexMessageProcessor {
             .unwrap_or_default()
     }
 
+    /// If a client sends `developer_instructions: null` during a mode switch,
+    /// use the built-in instructions for that mode.
+    fn normalize_turn_start_collaboration_mode(
+        &self,
+        mut collaboration_mode: CollaborationMode,
+    ) -> CollaborationMode {
+        if collaboration_mode.settings.developer_instructions.is_none()
+            && let Some(instructions) = self
+                .thread_manager
+                .list_collaboration_modes()
+                .into_iter()
+                .find(|preset| preset.mode == Some(collaboration_mode.mode))
+                .and_then(|preset| preset.developer_instructions.flatten())
+                .filter(|instructions| !instructions.is_empty())
+        {
+            collaboration_mode.settings.developer_instructions = Some(instructions);
+        }
+
+        collaboration_mode
+    }
+
     fn review_request_from_target(
         target: ApiReviewTarget,
     ) -> Result<(ReviewRequest, String), JSONRPCErrorError> {
@@ -4554,6 +4576,10 @@ impl CodexMessageProcessor {
             }
         };
 
+        let collaboration_mode = params
+            .collaboration_mode
+            .map(|mode| self.normalize_turn_start_collaboration_mode(mode));
+
         // Map v2 input items to core input items.
         let mapped_items: Vec<CoreInputItem> = params
             .input
@@ -4567,7 +4593,7 @@ impl CodexMessageProcessor {
             || params.model.is_some()
             || params.effort.is_some()
             || params.summary.is_some()
-            || params.collaboration_mode.is_some()
+            || collaboration_mode.is_some()
             || params.personality.is_some();
 
         // If any overrides are provided, update the session turn context first.
@@ -4581,7 +4607,7 @@ impl CodexMessageProcessor {
                     model: params.model,
                     effort: params.effort.map(Some),
                     summary: params.summary,
-                    collaboration_mode: params.collaboration_mode,
+                    collaboration_mode,
                     personality: params.personality,
                 })
                 .await;