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/codex.rs

@@ -121,6 +121,7 @@ use codex_protocol::request_user_input::RequestUserInputResponse;
 use codex_rmcp_client::ElicitationResponse;
 use codex_rmcp_client::OAuthCredentialsStoreMode;
 use codex_terminal_detection::user_agent;
+use codex_tools::filter_tool_suggest_discoverable_tools_for_client;
 use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_stream_parser::AssistantTextChunk;
 use codex_utils_stream_parser::AssistantTextStreamParser;
@@ -6546,7 +6547,7 @@ pub(crate) async fn built_tools(
             )
             .await
             .map(|discoverable_tools| {
-                crate::tools::discoverable::filter_tool_suggest_discoverable_tools_for_client(
+                filter_tool_suggest_discoverable_tools_for_client(
                     discoverable_tools,
                     turn_context.app_server_client_name.as_deref(),
                 )