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/context/world_state/top_level_tools.rs

@@ -107,10 +107,6 @@ impl TopLevelToolsState {
     }
 
     /// Track granular exec updates as part of this tool catalog.
-    #[cfg_attr(
-        not(test),
-        expect(dead_code, reason = "Activated by the follow-up runtime integration")
-    )]
     pub(crate) fn with_incremental_exec_tools(
         mut self,
         exec_definitions: Option<Arc<Vec<ExecDefinition>>>,