core: allow excluding tool namespaces from code mode (#26320)

## Why

Research and training setups need to control which tool namespaces
appear inside code mode's nested `tools` surface without disabling those
tools entirely. This makes it possible to train against a deliberately
reduced nested-tool setup while preserving the normal direct and
deferred tool paths.

## What

- Extend `features.code_mode` to accept structured configuration while
preserving the existing boolean syntax.
- Add an exact `excluded_tool_namespaces` list under
`[features.code_mode]`:

  ```toml
  [features.code_mode]
  enabled = true
  excluded_tool_namespaces = ["mcp__codex_apps", "multi_agent_v1"]
  ```

- Filter matching canonical `ToolName` namespaces when constructing code
mode's nested router and code-mode-specific direct tool descriptions.
- Keep excluded tools registered, directly exposed in mixed code mode,
and discoverable through top-level `tool_search` when otherwise
eligible.
- Derive deferred nested-tool guidance after namespace filtering so the
`exec` description does not advertise excluded-only deferred tools.
- Preserve the boolean/table representation when materializing config
locks and update the generated config schema.

## Testing

- `just test -p codex-features`
- `just test -p codex-config`
- `just test -p codex-core load_config_resolves_code_mode_config`
- `just test -p codex-core
lock_contains_prompts_and_materializes_features`
- `just test -p codex-core
excluded_deferred_namespaces_do_not_enable_nested_tool_guidance`
- `just test -p codex-core
code_mode_excludes_configured_nested_tool_namespaces`
- `cargo check -p codex-thread-manager-sample`

codex-rs/thread-manager-sample/src/main.rs

@@ -266,6 +266,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         web_search_mode: Constrained::allow_any(WebSearchMode::Disabled),
         web_search_config: None,
         experimental_request_user_input_enabled: true,
+        code_mode: Default::default(),
         use_experimental_unified_exec_tool: false,
         background_terminal_max_timeout: 300_000,
         ghost_snapshot: GhostSnapshotConfig::default(),