feat: expose multi-agent v2 as model-only tools (#22514)

## Why

`code_mode_only` filters code-mode nested tools out of the top-level
tool list. For multi-agent v2, we need a rollout shape where the
collaboration tools remain callable as normal model tools without also
being embedded into the code-mode `exec` tool declaration.

Related to this:
https://openai-corpws.slack.com/archives/C0AQLHB4U75/p1778660267922549

## What Changed

- Adds `features.multi_agent_v2.non_code_mode_only`, including config
resolution, profile override handling, and generated schema coverage.
- Introduces `ToolExposure::DirectModelOnly` so a tool can be included
in the initial model-visible list while staying out of the nested
code-mode tool surface.
- Applies that exposure to the multi-agent v2 tools when the new flag is
set: `spawn_agent`, `send_message`, `followup_task`, `wait_agent`,
`close_agent`, and `list_agents`.
- Updates code-mode-only filtering so direct-model-only tools remain
visible while ordinary nested code-mode tools are still hidden.

## Verification

- Added config parsing/profile tests for `non_code_mode_only`.
- Added tool spec coverage for the code-mode-only multi-agent v2
exposure behavior.

codex-rs/tools/src/tool_executor.rs

@@ -5,12 +5,30 @@ use crate::ToolName;
 use crate::ToolOutput;
 use crate::ToolSpec;
 
-/// Controls whether a tool is exposed in the initial model-visible tool list
-/// or registered for later discovery.
+/// Controls where a tool is exposed to the model.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum ToolExposure {
+    /// Include this tool in the initial model-visible tool list.
+    ///
+    /// When code mode is enabled, this tool is also available as a nested
+    /// code-mode tool.
     Direct,
+
+    /// Register this tool for later discovery, but omit it from the initial
+    /// model-visible tool list.
     Deferred,
+
+    /// Include this tool in the initial model-visible tool list only.
+    ///
+    /// In code-mode-only sessions, this keeps the tool callable as a normal
+    /// model tool while excluding it from the nested code-mode tool surface.
+    DirectModelOnly,
+}
+
+impl ToolExposure {
+    pub fn is_direct(self) -> bool {
+        matches!(self, Self::Direct | Self::DirectModelOnly)
+    }
 }
 
 /// Shared runtime contract for model-visible tools.