Extract tool discovery helpers into codex-tools

[bolinfest]

Why

Follow-up to #16379 and #16471.

codex-rs/core/src/tools/spec.rs still owned the pure discovery-shaping helpers that turn app metadata and discoverable tool metadata into the inputs used by tool_search and tool_suggest. Those helpers do not need codex-core runtime state, so keeping them in codex-core continued to blur the crate boundary this migration is trying to tighten.

This change keeps pushing spec-only logic behind the codex-tools API so codex-core can focus on wiring runtime handlers to the resulting tool definitions.

What Changed

• Added collect_tool_search_app_infos and collect_tool_suggest_entries to codex-rs/tools/src/tool_discovery.rs.
• Added a small ToolSearchAppSource adapter type in codex-tools so codex-core can pass app metadata into that shared helper logic without exposing ToolInfo across the crate boundary.
• Re-exported the new discovery helpers from codex-rs/tools/src/lib.rs, which remains exports-only.
• Updated codex-rs/core/src/tools/spec.rs to use those codex-tools helpers instead of maintaining local tool_search_app_infos and tool_suggest_entries functions.
• Removed the now-redundant helper implementations from codex-core.

Testing

• cargo test -p codex-tools
• cargo test -p codex-core tools::spec::tests