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_tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use codex_app_server_protocol::AppInfo;
 use pretty_assertions::assert_eq;
 use serde_json::json;
 use std::collections::BTreeMap;
@@ -147,3 +148,51 @@ fn discoverable_tool_enums_use_expected_wire_names() {
         })
     );
 }
+
+#[test]
+fn filter_tool_suggest_discoverable_tools_for_codex_tui_omits_plugins() {
+    let discoverable_tools = vec![
+        DiscoverableTool::Connector(Box::new(AppInfo {
+            id: "connector_google_calendar".to_string(),
+            name: "Google Calendar".to_string(),
+            description: Some("Plan events and schedules.".to_string()),
+            logo_url: None,
+            logo_url_dark: None,
+            distribution_channel: None,
+            branding: None,
+            app_metadata: None,
+            labels: None,
+            install_url: Some("https://example.test/google-calendar".to_string()),
+            is_accessible: false,
+            is_enabled: true,
+            plugin_display_names: Vec::new(),
+        })),
+        DiscoverableTool::Plugin(Box::new(DiscoverablePluginInfo {
+            id: "slack@openai-curated".to_string(),
+            name: "Slack".to_string(),
+            description: Some("Search Slack messages".to_string()),
+            has_skills: true,
+            mcp_server_names: vec!["slack".to_string()],
+            app_connector_ids: vec!["connector_slack".to_string()],
+        })),
+    ];
+
+    assert_eq!(
+        filter_tool_suggest_discoverable_tools_for_client(discoverable_tools, Some("codex-tui"),),
+        vec![DiscoverableTool::Connector(Box::new(AppInfo {
+            id: "connector_google_calendar".to_string(),
+            name: "Google Calendar".to_string(),
+            description: Some("Plan events and schedules.".to_string()),
+            logo_url: None,
+            logo_url_dark: None,
+            distribution_channel: None,
+            branding: None,
+            app_metadata: None,
+            labels: None,
+            install_url: Some("https://example.test/google-calendar".to_string()),
+            is_accessible: false,
+            is_enabled: true,
+            plugin_display_names: Vec::new(),
+        }))]
+    );
+}