codex-tools: extract discovery tool specs (#16193)

## Why

`core/src/tools/spec.rs` still owned the pure `tool_search` and
`tool_suggest` spec builders even though that logic no longer needed
`codex-core` runtime state. This change continues the `codex-tools`
migration by moving the reusable discovery and suggestion spec
construction out of `codex-core` so `spec.rs` is left with the
core-owned policy decisions about when these tools are exposed and what
metadata is available.

## What changed

- Added `codex-rs/tools/src/tool_discovery.rs` with the shared
`tool_search` and `tool_suggest` spec builders, plus focused unit tests
in `tool_discovery_tests.rs`.
- Moved the shared `DiscoverableToolAction` and `DiscoverableToolType`
declarations into `codex-tools` so the `tool_suggest` handler and the
extracted spec builders use the same wire-model enums.
- Updated `core/src/tools/spec.rs` to translate `ToolInfo` and
`DiscoverableTool` values into neutral `codex-tools` inputs and delegate
the actual spec building there.
- Removed the old template-based description rendering helpers from
`core/src/tools/spec.rs` and deleted the now-dead helper methods in
`core/src/tools/discoverable.rs`.
- Updated `codex-rs/tools/README.md` to document that discovery and
suggestion models/spec builders now live in `codex-tools`.

## Test plan

- `cargo test -p codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-discovery-specs cargo test -p
codex-core --lib tools::spec::`
- `CARGO_TARGET_DIR=/tmp/codex-core-discovery-specs cargo test -p
codex-core --lib tools::handlers::tool_suggest::`
- `just argument-comment-lint`

## References

- #16154
- #15923
- #15928
- #15944
- #15953
- #16031
- #16047
- #16129
- #16132
- #16138
- #16141

codex-rs/tools/src/lib.rs

@@ -13,6 +13,7 @@ mod mcp_tool;
 mod request_user_input_tool;
 mod responses_api;
 mod tool_definition;
+mod tool_discovery;
 mod tool_spec;
 mod utility_tool;
 mod view_image;
@@ -66,6 +67,12 @@ pub use responses_api::mcp_tool_to_deferred_responses_api_tool;
 pub use responses_api::mcp_tool_to_responses_api_tool;
 pub use responses_api::tool_definition_to_responses_api_tool;
 pub use tool_definition::ToolDefinition;
+pub use tool_discovery::DiscoverableToolAction;
+pub use tool_discovery::DiscoverableToolType;
+pub use tool_discovery::ToolSearchAppInfo;
+pub use tool_discovery::ToolSuggestEntry;
+pub use tool_discovery::create_tool_search_tool;
+pub use tool_discovery::create_tool_suggest_tool;
 pub use tool_spec::ConfiguredToolSpec;
 pub use tool_spec::ResponsesApiWebSearchFilters;
 pub use tool_spec::ResponsesApiWebSearchUserLocation;