[code mode] defer mcp tools from exec description

[sayan-oai]

Summary

• hide deferred MCP/app nested tool descriptions from the exec prompt in code mode
• add short guidance that omitted nested tools are still available through ALL_TOOLS
• cover the code_mode_only path with an integration test that discovers and calls a deferred app tool

Motivation

code_mode_only exposes only top-level exec/wait, but the exec description could still include a large nested-tool reference. This keeps deferred nested tools callable while avoiding that prompt bloat.

Tests

• just fmt
• just fix -p codex-code-mode
• just fix -p codex-tools
• cargo test -p codex-code-mode exec_description_mentions_deferred_nested_tools_when_available
• cargo test -p codex-tools create_code_mode_tool_matches_expected_spec
• cargo test -p codex-core code_mode_only_guides_all_tools_search_and_calls_deferred_app_tools