Preserve parent tool declarations in prediction forks (#57287)

## What changed

Keep inherited incremental tool declarations in MCP-disabled prediction forks and `thread/sample` requests, and append a developer notice that MCP tools are unavailable. Skip tool declaration updates within the inherited context window; new windows still declare the current tools.

Add the default-enabled `prediction_preserve_parent_tools` feature flag and its configuration schema entries.

## Testing

Update core snapshots and app-server tests to cover inherited declarations, the MCP-unavailable notice, preservation across MCP refresh, and continued parent tool availability.

GitOrigin-RevId: 3adf96c7d0e0093986dbdbd540bd47674a2c105c

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

@@ -1802,6 +1802,18 @@ impl Session {
                     }
                 }
                 self.persist_rollout_items(&rollout_items).await;
+                if self.preserve_parent_tool_declarations() {
+                    let notice = DeveloperInstructions::new(
+                        "<mcp_tools>\nMCP tools are unavailable for this request.\n</mcp_tools>",
+                    )
+                    .with_metadata(ContentItemMetadata::tool(/*namespace*/ None));
+                    self.record_conversation_items(
+                        &turn_context,
+                        turn_context.model_info(),
+                        &[ContextualUserFragment::into(notice)],
+                    )
+                    .await;
+                }
 
                 // Agent spawn owns its final durability barrier so it can overlap the edge write.
                 let persist_context = match self.fork_persistence {
@@ -4572,6 +4584,16 @@ impl Session {
         true
     }
 
+    fn preserve_parent_tool_declarations(&self) -> bool {
+        // Only prediction forks and thread/sample set this marker when they disable MCP.
+        self.enabled(Feature::PredictionPreserveParentTools)
+            && self
+                .services
+                .thread_extension_data
+                .get::<crate::McpServersDisabled>()
+                .is_some()
+    }
+
     pub fn enabled(&self, feature: Feature) -> bool {
         self.features.enabled(feature)
     }

codex-rs/core/src/session/world_state.rs

@@ -165,7 +165,9 @@ impl Session {
         let extension_metrics = super::extension_metrics::from_session_telemetry(
             step_context.session_telemetry.clone(),
         );
-        if incremental_tools {
+        let preserve_parent_tool_declarations =
+            !new_window && self.preserve_parent_tool_declarations();
+        if incremental_tools && !preserve_parent_tool_declarations {
             let specs = step_context.tool_router.model_visible_specs();
             let definitions = codex_tools::create_tools_json_for_responses_lite(&specs)?;
             world_state.add_section(TopLevelToolsState::new(