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/core/src/connectors.rs

@@ -18,6 +18,7 @@ use codex_connectors::AllConnectorsCacheKey;
 use codex_connectors::DirectoryListResponse;
 use codex_login::token_data::TokenData;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_tools::DiscoverableTool;
 use rmcp::model::ToolAnnotations;
 use serde::Deserialize;
 use serde::de::DeserializeOwned;
@@ -44,8 +45,6 @@ use crate::mcp_connection_manager::codex_apps_tools_cache_key;
 use crate::plugins::AppConnectorId;
 use crate::plugins::PluginsManager;
 use crate::plugins::list_tool_suggest_discoverable_plugins;
-use crate::tools::discoverable::DiscoverablePluginInfo;
-use crate::tools::discoverable::DiscoverableTool;
 use codex_features::Feature;
 
 pub use codex_connectors::CONNECTORS_CACHE_TTL;
@@ -133,7 +132,6 @@ pub(crate) async fn list_tool_suggest_discoverable_tools_with_auth(
     .map(DiscoverableTool::from);
     let discoverable_plugins = list_tool_suggest_discoverable_plugins(config)?
         .into_iter()
-        .map(DiscoverablePluginInfo::from)
         .map(DiscoverableTool::from);
     Ok(discoverable_connectors
         .chain(discoverable_plugins)