feat: surface multi-agent thread limit in spawn description (#19360)

## Summary
- Thread `agent_max_threads` into `ToolsConfig` and
`SpawnAgentToolOptions`.
- Render the configured `max_concurrent_threads_per_session` value in
the MultiAgentV2 `spawn_agent` description.
- Cover the description text in `codex-tools` unit tests and
`codex-core` tool spec tests.

## Validation
- `just fmt`
- `cargo test -p codex-tools`
- `cargo test -p codex-core spawn_agent_description`
- `git diff --check`

## Notes
- `cargo test -p codex-core` was also attempted, but unrelated
environment-sensitive tests failed with the active local environment.
Examples: approvals reviewer defaults observed `AutoReview` instead of
`User`, request-permissions event tests did not emit events, and
proxy-env tests saw `http://127.0.0.1:50604` from the active proxy
environment.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tools/src/agent_tool.rs

@@ -16,6 +16,7 @@ pub struct SpawnAgentToolOptions<'a> {
     pub hide_agent_type_model_reasoning: bool,
     pub include_usage_hint: bool,
     pub usage_hint_text: Option<String>,
+    pub max_concurrent_threads_per_session: Option<usize>,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -71,6 +72,7 @@ pub fn create_spawn_agent_tool_v2(options: SpawnAgentToolOptions<'_>) -> ToolSpe
             available_models_description.as_deref(),
             options.include_usage_hint,
             options.usage_hint_text,
+            options.max_concurrent_threads_per_session,
         ),
         strict: false,
         defer_loading: None,
@@ -655,8 +657,16 @@ fn spawn_agent_tool_description_v2(
     available_models_description: Option<&str>,
     include_usage_hint: bool,
     usage_hint_text: Option<String>,
+    max_concurrent_threads_per_session: Option<usize>,
 ) -> String {
     let agent_role_guidance = available_models_description.unwrap_or_default();
+    let concurrency_guidance = max_concurrent_threads_per_session
+        .map(|limit| {
+            format!(
+                "This session is configured with `max_concurrent_threads_per_session = {limit}` for concurrently open agent threads."
+            )
+        })
+        .unwrap_or_default();
 
     let tool_description = format!(
         r#"
@@ -666,7 +676,8 @@ You are then able to refer to this agent as `task_3` or `/root/task1/task_3` int
 The spawned agent will have the same tools as you and the ability to spawn its own subagents.
 {SPAWN_AGENT_INHERITED_MODEL_GUIDANCE}
 It will be able to send you and other running agents messages, and its final answer will be provided to you when it finishes.
-The new agent's canonical task name will be provided to it along with the message."#
+The new agent's canonical task name will be provided to it along with the message.
+{concurrency_guidance}"#
     );
 
     if !include_usage_hint {