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;

codex-rs/tools/src/tool_discovery.rs

@@ -14,6 +14,13 @@ pub struct ToolSearchAppInfo {
     pub description: Option<String>,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct ToolSearchAppSource<'a> {
+    pub server_name: &'a str,
+    pub connector_name: Option<&'a str>,
+    pub connector_description: Option<&'a str>,
+}
+
 #[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq)]
 #[serde(rename_all = "snake_case")]
 pub enum DiscoverableToolType {
@@ -178,6 +185,29 @@ pub fn create_tool_search_tool(app_tools: &[ToolSearchAppInfo], default_limit: u
     }
 }
 
+pub fn collect_tool_search_app_infos<'a>(
+    app_tools: impl IntoIterator<Item = ToolSearchAppSource<'a>>,
+    codex_apps_server_name: &str,
+) -> Vec<ToolSearchAppInfo> {
+    app_tools
+        .into_iter()
+        .filter(|tool| tool.server_name == codex_apps_server_name)
+        .filter_map(|tool| {
+            let name = tool
+                .connector_name
+                .map(str::trim)
+                .filter(|connector_name| !connector_name.is_empty())?
+                .to_string();
+            let description = tool
+                .connector_description
+                .map(str::trim)
+                .filter(|connector_description| !connector_description.is_empty())
+                .map(str::to_string);
+            Some(ToolSearchAppInfo { name, description })
+        })
+        .collect()
+}
+
 pub fn create_tool_suggest_tool(discoverable_tools: &[ToolSuggestEntry]) -> ToolSpec {
     let discoverable_tool_ids = discoverable_tools
         .iter()
@@ -245,6 +275,34 @@ pub fn create_tool_suggest_tool(discoverable_tools: &[ToolSuggestEntry]) -> Tool
     })
 }
 
+pub fn collect_tool_suggest_entries(
+    discoverable_tools: &[DiscoverableTool],
+) -> Vec<ToolSuggestEntry> {
+    discoverable_tools
+        .iter()
+        .map(|tool| match tool {
+            DiscoverableTool::Connector(connector) => ToolSuggestEntry {
+                id: connector.id.clone(),
+                name: connector.name.clone(),
+                description: connector.description.clone(),
+                tool_type: DiscoverableToolType::Connector,
+                has_skills: false,
+                mcp_server_names: Vec::new(),
+                app_connector_ids: Vec::new(),
+            },
+            DiscoverableTool::Plugin(plugin) => ToolSuggestEntry {
+                id: plugin.id.clone(),
+                name: plugin.name.clone(),
+                description: plugin.description.clone(),
+                tool_type: DiscoverableToolType::Plugin,
+                has_skills: plugin.has_skills,
+                mcp_server_names: plugin.mcp_server_names.clone(),
+                app_connector_ids: plugin.app_connector_ids.clone(),
+            },
+        })
+        .collect()
+}
+
 fn format_discoverable_tools(discoverable_tools: &[ToolSuggestEntry]) -> String {
     let mut discoverable_tools = discoverable_tools.to_vec();
     discoverable_tools.sort_by(|left, right| {