codex-tools: extract discoverable tool models (#16254)

## Why

`#16193` moved the pure `tool_search` and `tool_suggest` spec builders
into `codex-tools`, but `codex-core` still owned the shared
discoverable-tool model that those builders and the `tool_suggest`
runtime both depend on. This change continues the migration by moving
that reusable model boundary out of `codex-core` as well, so the
discovery/suggestion stack uses one shared set of types and
`core/src/tools` no longer needs its own `discoverable.rs` module.

## What changed

- Moved `DiscoverableTool`, `DiscoverablePluginInfo`, and
`filter_tool_suggest_discoverable_tools_for_client()` into
`codex-rs/tools/src/tool_discovery.rs` alongside the extracted
discovery/suggestion spec builders.
- Added `codex-app-server-protocol` as a `codex-tools` dependency so the
shared discoverable-tool model can own the connector-side `AppInfo`
variant directly.
- Updated `core/src/tools/handlers/tool_suggest.rs`,
`core/src/tools/spec.rs`, `core/src/tools/router.rs`,
`core/src/connectors.rs`, and `core/src/codex.rs` to consume the shared
`codex-tools` model instead of the old core-local declarations.
- Changed `core/src/plugins/discoverable.rs` to return
`DiscoverablePluginInfo` directly, moved the pure client-filter coverage
into `tool_discovery_tests.rs`, and deleted the old
`core/src/tools/discoverable.rs` module.
- Updated `codex-rs/tools/README.md` so the crate boundary documents
that `codex-tools` now owns the discoverable-tool models in addition to
the discovery/suggestion spec builders.

## Test plan

- `cargo test -p codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p
codex-core --lib tools::handlers::tool_suggest::`
- `CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p
codex-core --lib tools::spec::`
- `CARGO_TARGET_DIR=/tmp/codex-core-discoverable-model cargo test -p
codex-core --lib plugins::discoverable::`
- `just bazel-lock-check`
- `just argument-comment-lint`

## References

- #16193
- #16154
- #15923
- #15928
- #15944
- #15953
- #16031
- #16047
- #16129
- #16132
- #16138
- #16141

codex-rs/tools/src/tool_discovery.rs

@@ -1,10 +1,13 @@
 use crate::JsonSchema;
 use crate::ResponsesApiTool;
 use crate::ToolSpec;
+use codex_app_server_protocol::AppInfo;
 use serde::Deserialize;
 use serde::Serialize;
 use std::collections::BTreeMap;
 
+const TUI_CLIENT_NAME: &str = "codex-tui";
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct ToolSearchAppInfo {
     pub name: String,
@@ -34,6 +37,78 @@ pub enum DiscoverableToolAction {
     Enable,
 }
 
+#[derive(Clone, Debug, PartialEq)]
+pub enum DiscoverableTool {
+    Connector(Box<AppInfo>),
+    Plugin(Box<DiscoverablePluginInfo>),
+}
+
+impl DiscoverableTool {
+    pub fn tool_type(&self) -> DiscoverableToolType {
+        match self {
+            Self::Connector(_) => DiscoverableToolType::Connector,
+            Self::Plugin(_) => DiscoverableToolType::Plugin,
+        }
+    }
+
+    pub fn id(&self) -> &str {
+        match self {
+            Self::Connector(connector) => connector.id.as_str(),
+            Self::Plugin(plugin) => plugin.id.as_str(),
+        }
+    }
+
+    pub fn name(&self) -> &str {
+        match self {
+            Self::Connector(connector) => connector.name.as_str(),
+            Self::Plugin(plugin) => plugin.name.as_str(),
+        }
+    }
+
+    pub fn install_url(&self) -> Option<&str> {
+        match self {
+            Self::Connector(connector) => connector.install_url.as_deref(),
+            Self::Plugin(_) => None,
+        }
+    }
+}
+
+impl From<AppInfo> for DiscoverableTool {
+    fn from(value: AppInfo) -> Self {
+        Self::Connector(Box::new(value))
+    }
+}
+
+impl From<DiscoverablePluginInfo> for DiscoverableTool {
+    fn from(value: DiscoverablePluginInfo) -> Self {
+        Self::Plugin(Box::new(value))
+    }
+}
+
+pub fn filter_tool_suggest_discoverable_tools_for_client(
+    discoverable_tools: Vec<DiscoverableTool>,
+    app_server_client_name: Option<&str>,
+) -> Vec<DiscoverableTool> {
+    if app_server_client_name != Some(TUI_CLIENT_NAME) {
+        return discoverable_tools;
+    }
+
+    discoverable_tools
+        .into_iter()
+        .filter(|tool| !matches!(tool, DiscoverableTool::Plugin(_)))
+        .collect()
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct DiscoverablePluginInfo {
+    pub id: String,
+    pub name: String,
+    pub description: Option<String>,
+    pub has_skills: bool,
+    pub mcp_server_names: Vec<String>,
+    pub app_connector_ids: Vec<String>,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct ToolSuggestEntry {
     pub id: String,