code-mode: carry nested tool kind through runtime (#22377)

## Why

Code mode only used nested spec lookup at execution time to rediscover
whether a nested tool should be invoked as a function tool or a freeform
tool.

That information is already present in the enabled tool metadata that
code mode builds to expose `tools.*` and `ALL_TOOLS`, so re-looking it
up from the router was redundant and kept execution coupled to a
separate spec lookup path.

## What Changed

- thread `CodeModeToolKind` through the code-mode runtime `ToolCall`
event and `CodeModeNestedToolCall`
- emit the nested tool kind directly from the V8 callback using the
already-enabled tool metadata
- build nested tool payloads from the propagated kind instead of calling
`find_spec`
- remove the now-unused `find_spec` plumbing from the router and
parallel runtime helpers
- add unit coverage for function vs freeform payload shaping and update
affected router tests

## Testing

- `cargo test -p codex-code-mode`
- `cargo test -p codex-core code_mode::tests`
- `cargo test -p codex-core
extension_tool_bundles_are_model_visible_and_dispatchable`
- `cargo test -p codex-core
model_visible_specs_filter_deferred_dynamic_tools`

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

@@ -273,11 +273,6 @@ async fn model_visible_specs_filter_deferred_dynamic_tools() -> anyhow::Result<(
         },
     );
 
-    assert!(
-        router
-            .find_spec(&ToolName::namespaced("codex_app", hidden_tool))
-            .is_some()
-    );
     assert_eq!(
         namespace_function_names(&router.specs(), "codex_app"),
         vec![hidden_tool.to_string(), visible_tool.to_string()]
@@ -341,8 +336,9 @@ async fn extension_tool_bundles_are_model_visible_and_dispatchable() -> anyhow::
 
     assert!(
         router
-            .find_spec(&ToolName::plain("extension_echo"))
-            .is_some(),
+            .specs()
+            .iter()
+            .any(|spec| spec.name() == "extension_echo"),
         "expected extension-provided tool spec to be registered"
     );
     assert!(