Extract code-mode nested tool collection into codex-tools

[bolinfest]

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 ToolSpecs 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_