Fix code-mode deferred app tool routing

[dylan-hurd-oai]

Summary

• route code-mode nested tool calls through the nested-tool router instead of the outer model-facing router
• fixes code-mode-only turns where ALL_TOOLS advertises deferred app/MCP tools but invocation rejects them as not enabled

Test plan

• cargo test -p codex-core --test all suite::code_mode::code_mode_only_guides_all_tools_search_and_calls_deferred_app_tools -- --exact (3 consecutive passes)
• cargo test -p codex-core (hit unrelated stack overflow in tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed)
• just fmt
• just fix -p codex-core

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve app tool filtering for nested calls

When CodeMode runs with Apps enabled and an app or app tool is disabled by config, this new router rebuild bypasses the per-turn MCP exposure filtering in built_tools that calls codex_app_tool_is_enabled. build_nested_router registers the raw list_all_tools() output instead, so model-authored JavaScript can now dispatch disabled Codex app MCP tools through tools[...]/ALL_TOOLS rather than having the already-built filtered router reject them; the nested router should be built from the same filtered/deferred exposure set while still allowing eligible deferred tools.

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