Extract code-mode nested tool collection into codex-tools (#16509)

## Why
This is another small step in the `codex-core` -> `codex-tools`
migration described in `AGENTS.md`.

`core/src/tools/spec.rs` and `core/src/tools/code_mode/mod.rs` were both
hand-rolling the same pure transformation: convert visible `ToolSpec`s
into code-mode nested tool definitions, then sort and deduplicate by
tool name. That logic does not depend on core runtime state or handlers,
so keeping it in `codex-core` makes `spec.rs` harder to peel out later
than it needs to be.

## What Changed
- Add `collect_code_mode_tool_definitions()` to
`codex-rs/tools/src/code_mode.rs`.
- Reuse that helper from `codex-rs/core/src/tools/spec.rs` when
assembling the `exec` tool description.
- Reuse the same helper from `codex-rs/core/src/tools/code_mode/mod.rs`
when exposing nested tool metadata to the code-mode runtime.

This is intended to be a straight refactor with no behavior change and
no new test surface.

## Verification
- `cargo test -p codex-tools`
- `cargo test -p codex-core tools::spec::tests`
- `cargo test -p codex-core code_mode_only_`

codex-rs/core/src/tools/code_mode/mod.rs

@@ -28,7 +28,7 @@ use crate::tools::router::ToolRouterParams;
 use crate::unified_exec::resolve_max_tokens;
 use codex_features::Feature;
 use codex_tools::ToolSpec;
-use codex_tools::tool_spec_to_code_mode_tool_definition;
+use codex_tools::collect_code_mode_tool_definitions;
 use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_output_truncation::formatted_truncate_text_content_items_with_policy;
 use codex_utils_output_truncation::truncate_function_output_items_with_policy;
@@ -244,14 +244,8 @@ pub(super) async fn build_enabled_tools(
     exec: &ExecContext,
 ) -> Vec<codex_code_mode::ToolDefinition> {
     let router = build_nested_router(exec).await;
-    let mut out = router
-        .specs()
-        .into_iter()
-        .filter_map(|spec| tool_spec_to_code_mode_tool_definition(&spec))
-        .collect::<Vec<_>>();
-    out.sort_by(|left, right| left.name.cmp(&right.name));
-    out.dedup_by(|left, right| left.name == right.name);
-    out
+    let specs = router.specs();
+    collect_code_mode_tool_definitions(&specs)
 }
 
 async fn build_nested_router(exec: &ExecContext) -> ToolRouter {