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/tools/src/lib.rs

@@ -42,6 +42,7 @@ pub use apply_patch_tool::ApplyPatchToolArgs;
 pub use apply_patch_tool::create_apply_patch_freeform_tool;
 pub use apply_patch_tool::create_apply_patch_json_tool;
 pub use code_mode::augment_tool_spec_for_code_mode;
+pub use code_mode::collect_code_mode_tool_definitions;
 pub use code_mode::create_code_mode_tool;
 pub use code_mode::create_wait_tool;
 pub use code_mode::tool_spec_to_code_mode_tool_definition;