Emit one developer notice per incremental namespace update batch (#52295)

## What changed

Move incremental tool guidance out of namespace descriptions and into a developer message preceding each namespace declaration batch after the initial catalog. The notice covers newly added namespaces as well as changed ones, explaining that earlier tools remain available unless explicitly removed and that newer definitions replace earlier ones.

Retain the notice alongside tool declarations in full-history agent forks, and discard it when legacy compaction forks rebuild those declarations.

## Testing

Add unit coverage for multiple new namespaces sharing one notice. Update request-history snapshots for tool changes, removals and restoration, and regular Responses requests. Extend resume and fork tests to cover notice preservation without duplication and removal when rebuilding declarations.

GitOrigin-RevId: 05f79f5eb378a355cf935eabf63951015a9b454b

codex-rs/core/src/agent/control/spawn.rs

@@ -20,6 +20,7 @@ use crate::context::DeveloperInstructions;
 use crate::context::ManagedDeveloperInstructions;
 use crate::context::MultiAgentModeInstructions;
 use crate::context::MultiAgentRoleInstructions;
+use crate::context::world_state::IncrementalToolsHint;
 use crate::context::world_state::PersistentModeState;
 use crate::session::multi_agents::resolve_usage_hints;
 use crate::thread_manager::StartThreadOptions;
@@ -1150,7 +1151,10 @@ impl LocalAgentControl {
                     return false;
                 };
                 content.retain_mut(|content_item| {
-                    if content_item.kind().as_str() == BaseInstructionsFragment::KIND {
+                    if matches!(
+                        content_item.kind().as_str(),
+                        BaseInstructionsFragment::KIND | IncrementalToolsHint::KIND
+                    ) {
                         return preserve_context_baselines;
                     }
                     let ContentItem::InputText { text } = content_item.content_mut() else {