Add code_mode_only feature (#14617)

Summary
- add the code_mode_only feature flag/config schema and wire its
dependency on code_mode
- update code mode tool descriptions to list nested tools with detailed
headers
- restrict available tools for prompt and exec descriptions when
code_mode_only is enabled and test the behavior

Testing
- Not run (not requested)

codex-rs/core/src/codex.rs

@@ -6175,7 +6175,7 @@ fn build_prompt(
 ) -> Prompt {
     Prompt {
         input,
-        tools: router.specs(),
+        tools: router.model_visible_specs(),
         parallel_tool_calls: turn_context.model_info.supports_parallel_tool_calls,
         base_instructions,
         personality: turn_context.personality,