Preserve developer instruction annotations in subagent forks (#40297)

## What changed

- Add a dedicated contextual fragment for developer instructions that emits the
  `generic.developer_instructions` content kind.
- Use the fragment when adding child-specific developer instructions to a
  full-history subagent fork, preserving their classification and ensuring they
  reach the child exactly once without appearing in the parent request.

## Testing

- Extend full-history subagent coverage to verify the child-only instructions'
  annotation, placement, and count.

GitOrigin-RevId: 82589d311535225fed76d500a19458fda72f40f2

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

@@ -21,6 +21,7 @@ use crate::compact::CompactedHistoryMetadata;
 use crate::config::ManagedFeatures;
 use crate::config::resolve_tool_suggest_config_from_layer_stack;
 use crate::context::ContextualUserFragment;
+use crate::context::DeveloperInstructions;
 use crate::context::ManagedDeveloperInstructions;
 use crate::context::ModelSwitchInstructions;
 use crate::context::MultiAgentRoleInstructions;
@@ -3544,13 +3545,8 @@ impl Session {
             && let Some(developer_instructions) = turn_context.developer_instructions.as_deref()
             && !developer_instructions.is_empty()
         {
-            developer_sections.push(RenderedFragment::new(
-                "developer",
-                AnnotatedContent::input_text(
-                    developer_instructions,
-                    ContentItemKind("generic.developer_instructions".to_string()),
-                ),
-            ));
+            developer_sections
+                .push(DeveloperInstructions::new(developer_instructions).render_fragment());
         }
         let loaded_plugins = self
             .services