Wire incremental exec tool updates into Code Mode sessions (#60163)

## Why

Changes to nested tools should update the model's catalog without repeating the full `functions.exec` declaration or rewriting retained context.

## What changed

Pass structured exec definitions through the tool router into the session's tool world state. Enable granular updates for Responses Lite in `CodeModeOnly` when both `IncrementalTools` and `IncrementalExecTools` are enabled.

Append changed tool definitions, namespace instructions, and removals as developer messages. Keep common exec instructions and shared types frozen until compaction, then install the latest full catalog.

## Testing

Add feature-gating coverage and a scenario snapshot covering tool additions and removals, namespace instruction changes, eager/deferred transitions, re-enabling tools, retained context, and catalog restoration after compaction.

GitOrigin-RevId: bbf850d4e5018388eeb4c09345d885d776962e09

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

@@ -328,6 +328,7 @@ impl StepContext {
                 Vec::new(),
                 ToolMode::Direct,
                 BTreeMap::new(),
+                /*exec_definitions*/ None,
                 /*tool_namespaces_info*/ None,
                 &[],
             )),

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

@@ -208,11 +208,15 @@ impl Session {
                 .cloned()
                 .collect::<Vec<_>>();
             let definitions = codex_tools::create_tools_json_for_responses_lite(&specs)?;
-            world_state.add_section(TopLevelToolsState::new(
+            let mut top_level = TopLevelToolsState::new(
                 model_messages.incremental_tools(),
                 definitions,
                 Some(Arc::clone(&extension_metrics)),
-            )?);
+            )?;
+            if let Some(entries) = step_context.tool_router.exec_definitions() {
+                top_level = top_level.with_incremental_exec_tools(Some(entries));
+            }
+            world_state.add_section(top_level);
         }
         world_state.add_section(model_state);
         let token_budget_enabled = turn_context.config.features.enabled(Feature::TokenBudget)