Extract tool discovery helpers into codex-tools (#16477)

## 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`

codex-rs/tools/src/lib.rs

@@ -89,7 +89,10 @@ pub use tool_discovery::DiscoverableTool;
 pub use tool_discovery::DiscoverableToolAction;
 pub use tool_discovery::DiscoverableToolType;
 pub use tool_discovery::ToolSearchAppInfo;
+pub use tool_discovery::ToolSearchAppSource;
 pub use tool_discovery::ToolSuggestEntry;
+pub use tool_discovery::collect_tool_search_app_infos;
+pub use tool_discovery::collect_tool_suggest_entries;
 pub use tool_discovery::create_tool_search_tool;
 pub use tool_discovery::create_tool_suggest_tool;
 pub use tool_discovery::filter_tool_suggest_discoverable_tools_for_client;