Extract update_plan tool spec into codex-tools (#16481)

## Why

`codex-rs/core/src/tools/handlers/plan.rs` still owned both the
`update_plan` runtime handler and the static tool definition. The tool
definition is pure metadata, so keeping it in `codex-core` works against
the ongoing effort to move tool-spec code into `codex-tools` and keep
`codex-core` focused on orchestration and execution paths.

This continues the extraction work from #16379, #16471, and #16477.

## What Changed

- added `codex-rs/tools/src/plan_tool.rs` with
`create_update_plan_tool()`
- re-exported that constructor from `codex-rs/tools/src/lib.rs`
- updated `codex-rs/core/src/tools/spec.rs` and
`codex-rs/core/src/tools/spec_tests.rs` to use the `codex-tools` export
instead of a core-local static
- removed the old `PLAN_TOOL` definition from
`codex-rs/core/src/tools/handlers/plan.rs`; the `PlanHandler` runtime
logic still stays in `codex-core`
- tightened two `codex-core` aliases to `#[cfg(test)]` now that
production code no longer needs them

## Testing

- `cargo test -p codex-tools`
- `cargo test -p codex-core tools::spec::tests`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16481).
* #16482
* __->__ #16481

codex-rs/core/src/tools/spec_tests.rs

@@ -24,6 +24,7 @@ use codex_tools::ConfiguredToolSpec;
 use codex_tools::DiscoverablePluginInfo;
 use codex_tools::DiscoverableTool;
 use codex_tools::FreeformTool;
+use codex_tools::JsonSchema;
 use codex_tools::ResponsesApiTool;
 use codex_tools::ResponsesApiWebSearchFilters;
 use codex_tools::ResponsesApiWebSearchUserLocation;
@@ -41,6 +42,7 @@ use codex_tools::create_send_input_tool_v1;
 use codex_tools::create_send_message_tool;
 use codex_tools::create_spawn_agent_tool_v1;
 use codex_tools::create_spawn_agent_tool_v2;
+use codex_tools::create_update_plan_tool;
 use codex_tools::create_view_image_tool;
 use codex_tools::create_wait_agent_tool_v1;
 use codex_tools::create_wait_agent_tool_v2;
@@ -331,7 +333,7 @@ fn test_full_toolset_specs_for_gpt5_codex_unified_exec_web_search() {
             exec_permission_approvals_enabled: false,
         }),
         create_write_stdin_tool(),
-        PLAN_TOOL.clone(),
+        create_update_plan_tool(),
         request_user_input_tool_spec(/*default_mode_request_user_input*/ false),
         create_apply_patch_freeform_tool(),
         ToolSpec::WebSearch {